diff --git a/setup.py b/setup.py
index dd85db6a1..db47c25c7 100644
--- a/setup.py
+++ b/setup.py
@@ -52,6 +52,7 @@ from distutils.command.clean import clean
 
 compress_source = 'src/borg/compress.pyx'
 crypto_ll_source = 'src/borg/crypto/low_level.pyx'
+crypto_helpers = 'src/borg/crypto/_crypto_helpers.c'
 chunker_source = 'src/borg/chunker.pyx'
 hashindex_source = 'src/borg/hashindex.pyx'
 item_source = 'src/borg/item.pyx'
@@ -730,7 +731,7 @@ ext_modules = []
 if not on_rtd:
     ext_modules += [
     Extension('borg.compress', [compress_source], libraries=['lz4'], include_dirs=include_dirs, library_dirs=library_dirs, define_macros=define_macros),
-    Extension('borg.crypto.low_level', [crypto_ll_source], libraries=crypto_libraries, include_dirs=include_dirs, library_dirs=library_dirs, define_macros=define_macros),
+    Extension('borg.crypto.low_level', [crypto_ll_source, crypto_helpers], libraries=crypto_libraries, include_dirs=include_dirs, library_dirs=library_dirs, define_macros=define_macros),
     Extension('borg.hashindex', [hashindex_source]),
     Extension('borg.item', [item_source]),
     Extension('borg.chunker', [chunker_source]),
diff --git a/src/borg/archive.py b/src/borg/archive.py
index ff8f8729e..06470eaba 100644
--- a/src/borg/archive.py
+++ b/src/borg/archive.py
@@ -25,6 +25,7 @@ from .cache import ChunkListEntry
 from .crypto.key import key_factory
 from .compress import Compressor, CompressionSpec
 from .constants import *  # NOQA
+from .crypto.low_level import IntegrityError as IntegrityErrorBase
 from .hashindex import ChunkIndex, ChunkIndexEntry, CacheSynchronizer
 from .helpers import Manifest
 from .helpers import hardlinkable
@@ -1148,7 +1149,7 @@ class ArchiveChecker:
         else:
             try:
                 self.manifest, _ = Manifest.load(repository, (Manifest.Operation.CHECK,), key=self.key)
-            except IntegrityError as exc:
+            except IntegrityErrorBase as exc:
                 logger.error('Repository manifest is corrupted: %s', exc)
                 self.error_found = True
                 del self.chunks[Manifest.MANIFEST_ID]
@@ -1211,11 +1212,11 @@ class ArchiveChecker:
                 chunk_id = chunk_ids_revd.pop(-1)  # better efficiency
                 try:
                     encrypted_data = next(chunk_data_iter)
-                except (Repository.ObjectNotFound, IntegrityError) as err:
+                except (Repository.ObjectNotFound, IntegrityErrorBase) as err:
                     self.error_found = True
                     errors += 1
                     logger.error('chunk %s: %s', bin_to_hex(chunk_id), err)
-                    if isinstance(err, IntegrityError):
+                    if isinstance(err, IntegrityErrorBase):
                         defect_chunks.append(chunk_id)
                     # as the exception killed our generator, make a new one for remaining chunks:
                     if chunk_ids_revd:
@@ -1225,7 +1226,7 @@ class ArchiveChecker:
                     _chunk_id = None if chunk_id == Manifest.MANIFEST_ID else chunk_id
                     try:
                         self.key.decrypt(_chunk_id, encrypted_data)
-                    except IntegrityError as integrity_error:
+                    except IntegrityErrorBase as integrity_error:
                         self.error_found = True
                         errors += 1
                         logger.error('chunk %s, integrity error: %s', bin_to_hex(chunk_id), integrity_error)
@@ -1254,7 +1255,7 @@ class ArchiveChecker:
                         encrypted_data = self.repository.get(defect_chunk)
                         _chunk_id = None if defect_chunk == Manifest.MANIFEST_ID else defect_chunk
                         self.key.decrypt(_chunk_id, encrypted_data)
-                    except IntegrityError:
+                    except IntegrityErrorBase:
                         # failed twice -> get rid of this chunk
                         del self.chunks[defect_chunk]
                         self.repository.delete(defect_chunk)
@@ -1295,7 +1296,7 @@ class ArchiveChecker:
             cdata = self.repository.get(chunk_id)
             try:
                 data = self.key.decrypt(chunk_id, cdata)
-            except IntegrityError as exc:
+            except IntegrityErrorBase as exc:
                 logger.error('Skipping corrupted chunk: %s', exc)
                 self.error_found = True
                 continue
diff --git a/src/borg/crypto/_crypto_helpers.c b/src/borg/crypto/_crypto_helpers.c
new file mode 100644
index 000000000..0a433bb5f
--- /dev/null
+++ b/src/borg/crypto/_crypto_helpers.c
@@ -0,0 +1,35 @@
+/* some helpers, so our code also works with OpenSSL 1.0.x */
+
+#include <string.h>
+#include <openssl/opensslv.h>
+#include <openssl/hmac.h>
+
+#if OPENSSL_VERSION_NUMBER < 0x10100000L
+
+HMAC_CTX *HMAC_CTX_new(void)
+{
+   HMAC_CTX *ctx = OPENSSL_malloc(sizeof(*ctx));
+   if (ctx != NULL) {
+       memset(ctx, 0, sizeof *ctx);
+       HMAC_CTX_cleanup(ctx);
+   }
+   return ctx;
+}
+
+void HMAC_CTX_free(HMAC_CTX *ctx)
+{
+   if (ctx != NULL) {
+       HMAC_CTX_cleanup(ctx);
+       OPENSSL_free(ctx);
+   }
+}
+
+const EVP_CIPHER *EVP_aes_256_ocb(void){  /* dummy, so that code compiles */
+    return NULL;
+}
+
+const EVP_CIPHER *EVP_chacha20_poly1305(void){  /* dummy, so that code compiles */
+    return NULL;
+}
+
+#endif
diff --git a/src/borg/crypto/_crypto_helpers.h b/src/borg/crypto/_crypto_helpers.h
new file mode 100644
index 000000000..bb9afc418
--- /dev/null
+++ b/src/borg/crypto/_crypto_helpers.h
@@ -0,0 +1,15 @@
+/* some helpers, so our code also works with OpenSSL 1.0.x */
+
+#include <openssl/opensslv.h>
+#include <openssl/hmac.h>
+#include <openssl/evp.h>
+
+#if OPENSSL_VERSION_NUMBER < 0x10100000L
+
+HMAC_CTX *HMAC_CTX_new(void);
+void HMAC_CTX_free(HMAC_CTX *ctx);
+
+const EVP_CIPHER *EVP_aes_256_ocb(void);  /* dummy, so that code compiles */
+const EVP_CIPHER *EVP_chacha20_poly1305(void);  /* dummy, so that code compiles */
+
+#endif
diff --git a/src/borg/crypto/key.py b/src/borg/crypto/key.py
index 02cfed6e7..25aae6cf7 100644
--- a/src/borg/crypto/key.py
+++ b/src/borg/crypto/key.py
@@ -11,7 +11,7 @@ from hmac import HMAC, compare_digest
 
 import msgpack
 
-from borg.logger import create_logger
+from ..logger import create_logger
 
 logger = create_logger()
 
@@ -25,10 +25,10 @@ from ..helpers import get_limited_unpacker
 from ..helpers import bin_to_hex
 from ..item import Key, EncryptedKey
 from ..platform import SaveFile
-from .nonces import NonceManager
-from .low_level import AES, bytes_to_long, bytes_to_int, num_aes_blocks, hmac_sha256, blake2b_256, hkdf_hmac_sha512
 
-PREFIX = b'\0' * 8
+from .nonces import NonceManager
+from .low_level import AES, bytes_to_long, long_to_bytes, bytes_to_int, num_cipher_blocks, hmac_sha256, blake2b_256, hkdf_hmac_sha512
+from .low_level import AES256_CTR_HMAC_SHA256, AES256_CTR_BLAKE2b
 
 
 class PassphraseWrong(Error):
@@ -352,48 +352,31 @@ class AESKeyBase(KeyBase):
 
     PAYLOAD_OVERHEAD = 1 + 32 + 8  # TYPE + HMAC + NONCE
 
-    MAC = hmac_sha256
+    CIPHERSUITE = AES256_CTR_HMAC_SHA256
 
     logically_encrypted = True
 
     def encrypt(self, chunk):
         data = self.compressor.compress(chunk)
-        self.nonce_manager.ensure_reservation(num_aes_blocks(len(data)))
-        self.enc_cipher.reset()
-        data = b''.join((self.enc_cipher.iv[8:], self.enc_cipher.encrypt(data)))
-        assert (self.MAC is blake2b_256 and len(self.enc_hmac_key) == 128 or
-                self.MAC is hmac_sha256 and len(self.enc_hmac_key) == 32)
-        hmac = self.MAC(self.enc_hmac_key, data)
-        return b''.join((self.TYPE_STR, hmac, data))
+        next_iv = self.nonce_manager.ensure_reservation(self.cipher.next_iv(),
+                                                        self.cipher.block_count(len(data)))
+        return self.cipher.encrypt(data, header=self.TYPE_STR, iv=next_iv)
 
     def decrypt(self, id, data, decompress=True):
         if not (data[0] == self.TYPE or
             data[0] == PassphraseKey.TYPE and isinstance(self, RepoKey)):
             id_str = bin_to_hex(id) if id is not None else '(unknown)'
             raise IntegrityError('Chunk %s: Invalid encryption envelope' % id_str)
-        data_view = memoryview(data)
-        hmac_given = data_view[1:33]
-        assert (self.MAC is blake2b_256 and len(self.enc_hmac_key) == 128 or
-                self.MAC is hmac_sha256 and len(self.enc_hmac_key) == 32)
-        hmac_computed = memoryview(self.MAC(self.enc_hmac_key, data_view[33:]))
-        if not compare_digest(hmac_computed, hmac_given):
-            id_str = bin_to_hex(id) if id is not None else '(unknown)'
-            raise IntegrityError('Chunk %s: Encryption envelope checksum mismatch' % id_str)
-        self.dec_cipher.reset(iv=PREFIX + data[33:41])
-        payload = self.dec_cipher.decrypt(data_view[41:])
+        try:
+            payload = self.cipher.decrypt(data)
+        except IntegrityError as e:
+            raise IntegrityError("Chunk %s: Could not decrypt [%s]" % (bin_to_hex(id), str(e)))
         if not decompress:
             return payload
         data = self.decompress(payload)
         self.assert_id(id, data)
         return data
 
-    def extract_nonce(self, payload):
-        if not (payload[0] == self.TYPE or
-            payload[0] == PassphraseKey.TYPE and isinstance(self, RepoKey)):
-            raise IntegrityError('Manifest: Invalid encryption envelope')
-        nonce = bytes_to_long(payload[33:41])
-        return nonce
-
     def init_from_random_data(self, data=None):
         if data is None:
             data = os.urandom(100)
@@ -405,10 +388,21 @@ class AESKeyBase(KeyBase):
         if self.chunk_seed & 0x80000000:
             self.chunk_seed = self.chunk_seed - 0xffffffff - 1
 
-    def init_ciphers(self, manifest_nonce=0):
-        self.enc_cipher = AES(is_encrypt=True, key=self.enc_key, iv=manifest_nonce.to_bytes(16, byteorder='big'))
-        self.nonce_manager = NonceManager(self.repository, self.enc_cipher, manifest_nonce)
-        self.dec_cipher = AES(is_encrypt=False, key=self.enc_key)
+    def init_ciphers(self, manifest_data=None):
+        self.cipher = self.CIPHERSUITE(mac_key=self.enc_hmac_key, enc_key=self.enc_key, header_len=1, aad_offset=1)
+        if manifest_data is None:
+            nonce = 0
+        else:
+            if not (manifest_data[0] == self.TYPE or
+                    manifest_data[0] == PassphraseKey.TYPE and isinstance(self, RepoKey)):
+                raise IntegrityError('Manifest: Invalid encryption envelope')
+            # manifest_blocks is a safe upper bound on the amount of cipher blocks needed
+            # to encrypt the manifest. depending on the ciphersuite and overhead, it might
+            # be a bit too high, but that does not matter.
+            manifest_blocks = num_cipher_blocks(len(manifest_data))
+            nonce = self.cipher.extract_iv(manifest_data) + manifest_blocks
+        self.cipher.set_iv(nonce)
+        self.nonce_manager = NonceManager(self.repository, nonce)
 
 
 class Passphrase(str):
@@ -528,8 +522,7 @@ class PassphraseKey(ID_HMAC_SHA_256, AESKeyBase):
             key.init(repository, passphrase)
             try:
                 key.decrypt(None, manifest_data)
-                num_blocks = num_aes_blocks(len(manifest_data) - 41)
-                key.init_ciphers(key.extract_nonce(manifest_data) + num_blocks)
+                key.init_ciphers(manifest_data)
                 key._passphrase = passphrase
                 return key
             except IntegrityError:
@@ -568,8 +561,7 @@ class KeyfileKeyBase(AESKeyBase):
         else:
             if not key.load(target, passphrase):
                 raise PassphraseWrong
-        num_blocks = num_aes_blocks(len(manifest_data) - 41)
-        key.init_ciphers(key.extract_nonce(manifest_data) + num_blocks)
+        key.init_ciphers(manifest_data)
         key._passphrase = passphrase
         return key
 
@@ -604,7 +596,7 @@ class KeyfileKeyBase(AESKeyBase):
         assert enc_key.version == 1
         assert enc_key.algorithm == 'sha256'
         key = passphrase.kdf(enc_key.salt, enc_key.iterations, 32)
-        data = AES(is_encrypt=False, key=key).decrypt(enc_key.data)
+        data = AES(key, b'\0'*16).decrypt(enc_key.data)
         if hmac_sha256(key, data) == enc_key.hash:
             return data
 
@@ -613,7 +605,7 @@ class KeyfileKeyBase(AESKeyBase):
         iterations = PBKDF2_ITERATIONS
         key = passphrase.kdf(salt, iterations, 32)
         hash = hmac_sha256(key, data)
-        cdata = AES(is_encrypt=True, key=key).encrypt(data)
+        cdata = AES(key, b'\0'*16).encrypt(data)
         enc_key = EncryptedKey(
             version=1,
             salt=salt,
@@ -772,7 +764,7 @@ class Blake2KeyfileKey(ID_BLAKE2b_256, KeyfileKey):
     STORAGE = KeyBlobStorage.KEYFILE
 
     FILE_ID = 'BORG_KEY'
-    MAC = blake2b_256
+    CIPHERSUITE = AES256_CTR_BLAKE2b
 
 
 class Blake2RepoKey(ID_BLAKE2b_256, RepoKey):
@@ -781,7 +773,7 @@ class Blake2RepoKey(ID_BLAKE2b_256, RepoKey):
     ARG_NAME = 'repokey-blake2'
     STORAGE = KeyBlobStorage.REPO
 
-    MAC = blake2b_256
+    CIPHERSUITE = AES256_CTR_BLAKE2b
 
 
 class AuthenticatedKeyBase(RepoKey):
@@ -799,16 +791,9 @@ class AuthenticatedKeyBase(RepoKey):
         super().save(target, passphrase)
         self.logically_encrypted = False
 
-    def extract_nonce(self, payload):
-        # This is called during set-up of the AES ciphers we're not actually using for this
-        # key. Therefore the return value of this method doesn't matter; it's just around
-        # to not have it crash should key identification be run against a very small chunk
-        # by "borg check" when the manifest is lost. (The manifest is always large enough
-        # to have the original method read some garbage from bytes 33-41). (Also, the return
-        # value must be larger than the 41 byte bloat of the original format).
-        if payload[0] != self.TYPE:
+    def init_ciphers(self, manifest_data=None):
+        if manifest_data is not None and manifest_data[0] != self.TYPE:
             raise IntegrityError('Manifest: Invalid encryption envelope')
-        return 42
 
     def encrypt(self, chunk):
         data = self.compressor.compress(chunk)
@@ -816,7 +801,8 @@ class AuthenticatedKeyBase(RepoKey):
 
     def decrypt(self, id, data, decompress=True):
         if data[0] != self.TYPE:
-            raise IntegrityError('Chunk %s: Invalid envelope' % bin_to_hex(id))
+            id_str = bin_to_hex(id) if id is not None else '(unknown)'
+            raise IntegrityError('Chunk %s: Invalid envelope' % id_str)
         payload = memoryview(data)[1:]
         if not decompress:
             return payload
diff --git a/src/borg/crypto/low_level.pyx b/src/borg/crypto/low_level.pyx
index a68cd820f..da06c73e2 100644
--- a/src/borg/crypto/low_level.pyx
+++ b/src/borg/crypto/low_level.pyx
@@ -1,15 +1,51 @@
-"""A thin OpenSSL wrapper"""
+"""An AEAD style OpenSSL wrapper
+
+API:
+
+    encrypt(data, header=b'', aad_offset=0) -> envelope
+    decrypt(envelope, header_len=0, aad_offset=0) -> data
+
+Envelope layout:
+
+|<--------------------------- envelope ------------------------------------------>|
+|<------------ header ----------->|<---------- ciphersuite specific ------------->|
+|<-- not auth data -->|<-- aad -->|<-- e.g.:  S(aad, iv, E(data)), iv, E(data) -->|
+
+|--- #aad_offset ---->|
+|------------- #header_len ------>|
+
+S means a cryptographic signature function (like HMAC or GMAC).
+E means a encryption function (like AES).
+iv is the initialization vector / nonce, if needed.
+
+The split of header into not authenticated data and aad (additional authenticated
+data) is done to support the legacy envelope layout as used in attic and early borg
+(where the TYPE byte was not authenticated) and avoid unneeded memcpy and string
+garbage.
+
+Newly designed envelope layouts can just authenticate the whole header.
+
+IV handling:
+
+    iv = ...  # just never repeat!
+    cs = CS(hmac_key, enc_key, iv=iv)
+    envelope = cs.encrypt(data, header, aad_offset)
+    iv = cs.next_iv(len(data))
+    (repeat)
+"""
 
 import hashlib
 import hmac
 from math import ceil
 
-from libc.stdlib cimport malloc, free
+from cpython cimport PyMem_Malloc, PyMem_Free
 from cpython.buffer cimport PyBUF_SIMPLE, PyObject_GetBuffer, PyBuffer_Release
 from cpython.bytes cimport PyBytes_FromStringAndSize
 
 API_VERSION = '1.1_02'
 
+cdef extern from "openssl/crypto.h":
+    int CRYPTO_memcmp(const void *a, const void *b, size_t len)
 
 cdef extern from "../algorithms/blake2-libselect.h":
     ctypedef struct blake2b_state:
@@ -29,9 +65,14 @@ cdef extern from "openssl/evp.h":
         pass
     ctypedef struct ENGINE:
         pass
+
     const EVP_CIPHER *EVP_aes_256_ctr()
-    EVP_CIPHER_CTX *EVP_CIPHER_CTX_new()
-    void EVP_CIPHER_CTX_free(EVP_CIPHER_CTX *a)
+    const EVP_CIPHER *EVP_aes_256_gcm()
+    const EVP_CIPHER *EVP_aes_256_ocb()
+    const EVP_CIPHER *EVP_chacha20_poly1305()
+
+    void EVP_CIPHER_CTX_init(EVP_CIPHER_CTX *a)
+    void EVP_CIPHER_CTX_cleanup(EVP_CIPHER_CTX *a)
 
     int EVP_EncryptInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *cipher, ENGINE *impl,
                            const unsigned char *key, const unsigned char *iv)
@@ -44,58 +85,83 @@ cdef extern from "openssl/evp.h":
     int EVP_EncryptFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl)
     int EVP_DecryptFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl)
 
-    EVP_MD *EVP_sha256() nogil
+    int EVP_CIPHER_CTX_ctrl(EVP_CIPHER_CTX *ctx, int type, int arg, void *ptr)
+    int EVP_CTRL_GCM_GET_TAG
+    int EVP_CTRL_GCM_SET_TAG
+    int EVP_CTRL_GCM_SET_IVLEN
 
+    const EVP_MD *EVP_sha256() nogil
+
+    EVP_CIPHER_CTX *EVP_CIPHER_CTX_new()
+    void EVP_CIPHER_CTX_free(EVP_CIPHER_CTX *a)
 
 cdef extern from "openssl/hmac.h":
+    ctypedef struct HMAC_CTX:
+        pass
+
+    void HMAC_CTX_init(HMAC_CTX *ctx)
+    void HMAC_CTX_cleanup(HMAC_CTX *ctx)
+
+    HMAC_CTX *HMAC_CTX_new()
+    void HMAC_CTX_free(HMAC_CTX *a)
+
+    int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len, const EVP_MD *md, ENGINE *impl)
+    int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len)
+    int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len)
+
     unsigned char *HMAC(const EVP_MD *evp_md,
                     const void *key, int key_len,
                     const unsigned char *data, int data_len,
                     unsigned char *md, unsigned int *md_len) nogil
 
+cdef extern from "_crypto_helpers.h":
+    long OPENSSL_VERSION_NUMBER
+
+    ctypedef struct HMAC_CTX:
+        pass
+
+    HMAC_CTX *HMAC_CTX_new()
+    void HMAC_CTX_free(HMAC_CTX *a)
+
+    const EVP_CIPHER *EVP_aes_256_ocb()  # dummy
+    const EVP_CIPHER *EVP_chacha20_poly1305()  # dummy
+
+
+openssl10 = OPENSSL_VERSION_NUMBER < 0x10100000
+
+
 import struct
 
 _int = struct.Struct('>I')
 _long = struct.Struct('>Q')
-_2long = struct.Struct('>QQ')
 
 bytes_to_int = lambda x, offset=0: _int.unpack_from(x, offset)[0]
 bytes_to_long = lambda x, offset=0: _long.unpack_from(x, offset)[0]
 long_to_bytes = lambda x: _long.pack(x)
 
 
-def bytes16_to_int(b, offset=0):
-    h, l = _2long.unpack_from(b, offset)
-    return (h << 64) + l
+def num_cipher_blocks(length, blocksize=16):
+    """Return the number of cipher blocks required to encrypt/decrypt <length> bytes of data.
 
+    For a precise computation, <blocksize> must be the used cipher's block size (AES: 16, CHACHA20: 64).
 
-def int_to_bytes16(i):
-    max_uint64 = 0xffffffffffffffff
-    l = i & max_uint64
-    h = (i >> 64) & max_uint64
-    return _2long.pack(h, l)
+    For a safe-upper-boundary computation, <blocksize> must be the MINIMUM of the block sizes (in
+    bytes) of ALL supported ciphers. This can be used to adjust a counter if the used cipher is not
+    known (yet).
+    The default value of blocksize must be adjusted so it reflects this minimum, so a call of this
+    function without a blocksize is "safe-upper-boundary by default".
 
-
-def increment_iv(iv, amount=1):
+    Padding cipher modes are not supported.
     """
-    Increment the IV by the given amount (default 1).
-
-    :param iv: input IV, 16 bytes (128 bit)
-    :param amount: increment value
-    :return: input_IV + amount, 16 bytes (128 bit)
-    """
-    assert len(iv) == 16
-    iv = bytes16_to_int(iv)
-    iv += amount
-    iv = int_to_bytes16(iv)
-    return iv
+    return (length + blocksize - 1) // blocksize
 
 
-def num_aes_blocks(int length):
-    """Return the number of AES blocks required to encrypt/decrypt *length* bytes of data.
-       Note: this is only correct for modes without padding, like AES-CTR.
-    """
-    return (length + 15) // 16
+class CryptoError(Exception):
+    """Malfunction in the crypto module."""
+
+
+class IntegrityError(CryptoError):
+    """Integrity checks failed. Corrupted or tampered data."""
 
 
 cdef Py_buffer ro_buffer(object data) except *:
@@ -104,101 +170,641 @@ cdef Py_buffer ro_buffer(object data) except *:
     return view
 
 
-cdef class AES:
-    """A thin wrapper around the OpenSSL EVP cipher API
-    """
+class UNENCRYPTED:
+    # Layout: HEADER + PlainText
+
+    def __init__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        assert mac_key is None
+        assert enc_key is None
+        self.header_len = header_len
+        self.set_iv(iv)
+
+    def encrypt(self, data, header=b'', iv=None):
+        """
+        IMPORTANT: it is called encrypt to satisfy the crypto api naming convention,
+        but this does NOT encrypt and it does NOT compute and store a MAC either.
+        """
+        if iv is not None:
+            self.set_iv(iv)
+        assert self.iv is not None, 'iv needs to be set before encrypt is called'
+        return header + data
+
+    def decrypt(self, envelope):
+        """
+        IMPORTANT: it is called decrypt to satisfy the crypto api naming convention,
+        but this does NOT decrypt and it does NOT verify a MAC either, because data
+        is not encrypted and there is no MAC.
+        """
+        return memoryview(envelope)[self.header_len:]
+
+    def block_count(self, length):
+        return 0
+
+    def set_iv(self, iv):
+        self.iv = iv
+
+    def next_iv(self):
+        return self.iv
+
+    def extract_iv(self, envelope):
+        return 0
+
+
+cdef class AES256_CTR_BASE:
+    # Layout: HEADER + MAC 32 + IV 8 + CT (same as attic / borg < 1.2 IF HEADER = TYPE_BYTE, no AAD)
+
     cdef EVP_CIPHER_CTX *ctx
-    cdef int is_encrypt
-    cdef unsigned char iv_orig[16]
+    cdef unsigned char *enc_key
+    cdef int cipher_blk_len
+    cdef int iv_len, iv_len_short
+    cdef int aad_offset
+    cdef int header_len
+    cdef int mac_len
+    cdef unsigned char iv[16]
     cdef long long blocks
 
-    def __cinit__(self, is_encrypt, key, iv=None):
+    @staticmethod
+    def requirements_check():
+        if OPENSSL_VERSION_NUMBER < 0x10000000:
+            raise ValueError('AES CTR requires OpenSSL >= 1.0.0. Detected: OpenSSL %08x' % OPENSSL_VERSION_NUMBER)
+
+    def __init__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        self.requirements_check()
+        assert isinstance(enc_key, bytes) and len(enc_key) == 32
+        self.cipher_blk_len = 16
+        self.iv_len = sizeof(self.iv)
+        self.iv_len_short = 8
+        assert aad_offset <= header_len
+        self.aad_offset = aad_offset
+        self.header_len = header_len
+        self.mac_len = 32
+        self.enc_key = enc_key
+        if iv is not None:
+            self.set_iv(iv)
+        else:
+            self.blocks = -1  # make sure set_iv is called before encrypt
+
+    def __cinit__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
         self.ctx = EVP_CIPHER_CTX_new()
-        self.is_encrypt = is_encrypt
-        # Set cipher type and mode
-        cipher_mode = EVP_aes_256_ctr()
-        if self.is_encrypt:
-            if not EVP_EncryptInit_ex(self.ctx, cipher_mode, NULL, NULL, NULL):
-                raise Exception('EVP_EncryptInit_ex failed')
-        else:  # decrypt
-            if not EVP_DecryptInit_ex(self.ctx, cipher_mode, NULL, NULL, NULL):
-                raise Exception('EVP_DecryptInit_ex failed')
-        self.reset(key, iv)
 
     def __dealloc__(self):
         EVP_CIPHER_CTX_free(self.ctx)
 
-    def reset(self, key=None, iv=None):
-        cdef const unsigned char *key2 = NULL
-        cdef const unsigned char *iv2 = NULL
-        if key:
-            key2 = key
-        if iv:
-            iv2 = iv
-            assert isinstance(iv, bytes) and len(iv) == 16
-            for i in range(16):
-                self.iv_orig[i] = iv[i]
-            self.blocks = 0  # number of AES blocks encrypted starting with iv_orig
-        # Initialise key and IV
-        if self.is_encrypt:
-            if not EVP_EncryptInit_ex(self.ctx, NULL, NULL, key2, iv2):
-                raise Exception('EVP_EncryptInit_ex failed')
-        else:  # decrypt
-            if not EVP_DecryptInit_ex(self.ctx, NULL, NULL, key2, iv2):
-                raise Exception('EVP_DecryptInit_ex failed')
+    cdef mac_compute(self, const unsigned char *data1, int data1_len,
+                     const unsigned char *data2, int data2_len,
+                     const unsigned char *mac_buf):
+        raise NotImplementedError
 
-    @property
-    def iv(self):
-        return increment_iv(self.iv_orig[:16], self.blocks)
+    cdef mac_verify(self, const unsigned char *data1, int data1_len,
+                    const unsigned char *data2, int data2_len,
+                    const unsigned char *mac_buf, const unsigned char *mac_wanted):
+        raise NotImplementedError
 
-    def encrypt(self, data):
-        cdef Py_buffer data_buf = ro_buffer(data)
-        cdef int inl = len(data)
-        cdef int ctl = 0
-        cdef int outl = 0
-        # note: modes that use padding, need up to one extra AES block (16b)
-        cdef unsigned char *out = <unsigned char *>malloc(inl+16)
-        if not out:
+    def encrypt(self, data, header=b'', iv=None):
+        """
+        encrypt data, compute mac over aad + iv + cdata, prepend header.
+        aad_offset is the offset into the header where aad starts.
+        """
+        if iv is not None:
+            self.set_iv(iv)
+        assert self.blocks == 0, 'iv needs to be set before encrypt is called'
+        cdef int ilen = len(data)
+        cdef int hlen = len(header)
+        assert hlen == self.header_len
+        cdef int aoffset = self.aad_offset
+        cdef int alen = hlen - aoffset
+        cdef unsigned char *odata = <unsigned char *>PyMem_Malloc(hlen + self.mac_len + self.iv_len_short +
+                                                                  ilen + self.cipher_blk_len)  # play safe, 1 extra blk
+        if not odata:
+            raise MemoryError
+        cdef int olen
+        cdef int offset
+        cdef Py_buffer idata = ro_buffer(data)
+        cdef Py_buffer hdata = ro_buffer(header)
+        try:
+            offset = 0
+            for i in range(hlen):
+                odata[offset+i] = header[i]
+            offset += hlen
+            offset += self.mac_len
+            self.store_iv(odata+offset, self.iv)
+            offset += self.iv_len_short
+            rc = EVP_EncryptInit_ex(self.ctx, EVP_aes_256_ctr(), NULL, self.enc_key, self.iv)
+            if not rc:
+                raise CryptoError('EVP_EncryptInit_ex failed')
+            rc = EVP_EncryptUpdate(self.ctx, odata+offset, &olen, <const unsigned char*> idata.buf, ilen)
+            if not rc:
+                raise CryptoError('EVP_EncryptUpdate failed')
+            offset += olen
+            rc = EVP_EncryptFinal_ex(self.ctx, odata+offset, &olen)
+            if not rc:
+                raise CryptoError('EVP_EncryptFinal_ex failed')
+            offset += olen
+            self.mac_compute(<const unsigned char *> hdata.buf+aoffset, alen,
+                              odata+hlen+self.mac_len, offset-hlen-self.mac_len,
+                              odata+hlen)
+            self.blocks += self.block_count(ilen)
+            return odata[:offset]
+        finally:
+            PyMem_Free(odata)
+            PyBuffer_Release(&hdata)
+            PyBuffer_Release(&idata)
+
+    def decrypt(self, envelope):
+        """
+        authenticate aad + iv + cdata, decrypt cdata, ignore header bytes up to aad_offset.
+        """
+        cdef int ilen = len(envelope)
+        cdef int hlen = self.header_len
+        assert hlen == self.header_len
+        cdef int aoffset = self.aad_offset
+        cdef int alen = hlen - aoffset
+        cdef unsigned char *odata = <unsigned char *>PyMem_Malloc(ilen + self.cipher_blk_len)  # play safe, 1 extra blk
+        if not odata:
+            raise MemoryError
+        cdef int olen
+        cdef int offset
+        cdef unsigned char mac_buf[32]
+        assert sizeof(mac_buf) == self.mac_len
+        cdef Py_buffer idata = ro_buffer(envelope)
+        try:
+            self.mac_verify(<const unsigned char *> idata.buf+aoffset, alen,
+                             <const unsigned char *> idata.buf+hlen+self.mac_len, ilen-hlen-self.mac_len,
+                             mac_buf, <const unsigned char *> idata.buf+hlen)
+            iv = self.fetch_iv(<unsigned char *> idata.buf+hlen+self.mac_len)
+            self.set_iv(iv)
+            if not EVP_DecryptInit_ex(self.ctx, EVP_aes_256_ctr(), NULL, self.enc_key, iv):
+                raise CryptoError('EVP_DecryptInit_ex failed')
+            offset = 0
+            rc = EVP_DecryptUpdate(self.ctx, odata+offset, &olen,
+                                   <const unsigned char*> idata.buf+hlen+self.mac_len+self.iv_len_short,
+                                   ilen-hlen-self.mac_len-self.iv_len_short)
+            if not rc:
+                raise CryptoError('EVP_DecryptUpdate failed')
+            offset += olen
+            rc = EVP_DecryptFinal_ex(self.ctx, odata+offset, &olen)
+            if rc <= 0:
+                raise CryptoError('EVP_DecryptFinal_ex failed')
+            offset += olen
+            self.blocks += self.block_count(offset)
+            return odata[:offset]
+        finally:
+            PyMem_Free(odata)
+            PyBuffer_Release(&idata)
+
+    def block_count(self, length):
+        return num_cipher_blocks(length, self.cipher_blk_len)
+
+    def set_iv(self, iv):
+        # set_iv needs to be called before each encrypt() call
+        if isinstance(iv, int):
+            iv = iv.to_bytes(self.iv_len, byteorder='big')
+        assert isinstance(iv, bytes) and len(iv) == self.iv_len
+        for i in range(self.iv_len):
+            self.iv[i] = iv[i]
+        self.blocks = 0  # how many AES blocks got encrypted with this IV?
+
+    def next_iv(self):
+        # call this after encrypt() to get the next iv (int) for the next encrypt() call
+        iv = int.from_bytes(self.iv[:self.iv_len], byteorder='big')
+        return iv + self.blocks
+
+    cdef fetch_iv(self, unsigned char * iv_in):
+        # fetch lower self.iv_len_short bytes of iv and add upper zero bytes
+        return b'\0' * (self.iv_len - self.iv_len_short) + iv_in[0:self.iv_len_short]
+
+    cdef store_iv(self, unsigned char * iv_out, unsigned char * iv):
+        # store only lower self.iv_len_short bytes, upper bytes are assumed to be 0
+        cdef int i
+        for i in range(self.iv_len_short):
+            iv_out[i] = iv[(self.iv_len-self.iv_len_short)+i]
+
+    def extract_iv(self, envelope):
+        offset = self.header_len + self.mac_len
+        return bytes_to_long(envelope[offset:offset+self.iv_len_short])
+
+
+cdef class AES256_CTR_HMAC_SHA256(AES256_CTR_BASE):
+    cdef HMAC_CTX *hmac_ctx
+    cdef unsigned char *mac_key
+
+    def __init__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        assert isinstance(mac_key, bytes) and len(mac_key) == 32
+        self.mac_key = mac_key
+        super().__init__(mac_key, enc_key, iv=iv, header_len=header_len, aad_offset=aad_offset)
+
+    def __cinit__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        self.hmac_ctx = HMAC_CTX_new()
+
+    def __dealloc__(self):
+        HMAC_CTX_free(self.hmac_ctx)
+
+    cdef mac_compute(self, const unsigned char *data1, int data1_len,
+                     const unsigned char *data2, int data2_len,
+                     const unsigned char *mac_buf):
+        if not HMAC_Init_ex(self.hmac_ctx, self.mac_key, self.mac_len, EVP_sha256(), NULL):
+            raise CryptoError('HMAC_Init_ex failed')
+        if not HMAC_Update(self.hmac_ctx, data1, data1_len):
+            raise CryptoError('HMAC_Update failed')
+        if not HMAC_Update(self.hmac_ctx, data2, data2_len):
+            raise CryptoError('HMAC_Update failed')
+        if not HMAC_Final(self.hmac_ctx, mac_buf, NULL):
+            raise CryptoError('HMAC_Final failed')
+
+    cdef mac_verify(self, const unsigned char *data1, int data1_len,
+                    const unsigned char *data2, int data2_len,
+                    const unsigned char *mac_buf, const unsigned char *mac_wanted):
+        self.mac_compute(data1, data1_len, data2, data2_len, mac_buf)
+        if CRYPTO_memcmp(mac_buf, mac_wanted, self.mac_len):
+            raise IntegrityError('MAC Authentication failed')
+
+
+cdef class AES256_CTR_BLAKE2b(AES256_CTR_BASE):
+    cdef unsigned char *mac_key
+
+    def __init__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        assert isinstance(mac_key, bytes) and len(mac_key) == 128
+        self.mac_key = mac_key
+        super().__init__(mac_key, enc_key, iv=iv, header_len=header_len, aad_offset=aad_offset)
+
+    def __cinit__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        pass
+
+    def __dealloc__(self):
+        pass
+
+    cdef mac_compute(self, const unsigned char *data1, int data1_len,
+                     const unsigned char *data2, int data2_len,
+                     const unsigned char *mac_buf):
+        cdef blake2b_state state
+        cdef int rc
+        rc = blake2b_init(&state, self.mac_len)
+        if rc == -1:
+            raise Exception('blake2b_init() failed')
+        with nogil:
+            rc = blake2b_update(&state, self.mac_key, 128)
+            if rc != -1:
+                rc = blake2b_update(&state, data1, data1_len)
+                if rc != -1:
+                    rc = blake2b_update(&state, data2, data2_len)
+        if rc == -1:
+            raise Exception('blake2b_update() failed')
+        rc = blake2b_final(&state, mac_buf, self.mac_len)
+        if rc == -1:
+            raise Exception('blake2b_final() failed')
+
+    cdef mac_verify(self, const unsigned char *data1, int data1_len,
+                    const unsigned char *data2, int data2_len,
+                    const unsigned char *mac_buf, const unsigned char *mac_wanted):
+        self.mac_compute(data1, data1_len, data2, data2_len, mac_buf)
+        if CRYPTO_memcmp(mac_buf, mac_wanted, self.mac_len):
+            raise IntegrityError('MAC Authentication failed')
+
+
+ctypedef const EVP_CIPHER * (* CIPHER)()
+
+
+cdef class _AEAD_BASE:
+    # Layout: HEADER + MAC 16 + IV 12 + CT
+
+    cdef CIPHER cipher
+    cdef EVP_CIPHER_CTX *ctx
+    cdef unsigned char *enc_key
+    cdef int cipher_blk_len
+    cdef int iv_len
+    cdef int aad_offset
+    cdef int header_len
+    cdef int mac_len
+    cdef unsigned char iv[12]
+    cdef long long blocks
+
+    @staticmethod
+    def requirements_check():
+        """check whether library requirements for this ciphersuite are satisfied"""
+        raise NotImplemented  # override / implement in child class
+
+    def __init__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        assert mac_key is None
+        assert isinstance(enc_key, bytes) and len(enc_key) == 32
+        self.iv_len = sizeof(self.iv)
+        self.header_len = 1
+        assert aad_offset <= header_len
+        self.aad_offset = aad_offset
+        self.header_len = header_len
+        self.mac_len = 16
+        self.enc_key = enc_key
+        if iv is not None:
+            self.set_iv(iv)
+        else:
+            self.blocks = -1  # make sure set_iv is called before encrypt
+
+    def __cinit__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        self.ctx = EVP_CIPHER_CTX_new()
+
+    def __dealloc__(self):
+        EVP_CIPHER_CTX_free(self.ctx)
+
+    def encrypt(self, data, header=b'', iv=None):
+        """
+        encrypt data, compute mac over aad + iv + cdata, prepend header.
+        aad_offset is the offset into the header where aad starts.
+        """
+        if iv is not None:
+            self.set_iv(iv)
+        assert self.blocks == 0, 'iv needs to be set before encrypt is called'
+        # AES-GCM, AES-OCB, CHACHA20 ciphers all add a internal 32bit counter to the 96bit (12Byte)
+        # IV we provide, thus we must not encrypt more than 2^32 cipher blocks with same IV).
+        block_count = self.block_count(len(data))
+        if block_count > 2**32:
+            raise ValueError('too much data, would overflow internal 32bit counter')
+        cdef int ilen = len(data)
+        cdef int hlen = len(header)
+        assert hlen == self.header_len
+        cdef int aoffset = self.aad_offset
+        cdef int alen = hlen - aoffset
+        cdef unsigned char *odata = <unsigned char *>PyMem_Malloc(hlen + self.mac_len + self.iv_len +
+                                                                  ilen + self.cipher_blk_len)
+        if not odata:
+            raise MemoryError
+        cdef int olen
+        cdef int offset
+        cdef Py_buffer idata = ro_buffer(data)
+        cdef Py_buffer hdata = ro_buffer(header)
+        try:
+            offset = 0
+            for i in range(hlen):
+                odata[offset+i] = header[i]
+            offset += hlen
+            offset += self.mac_len
+            self.store_iv(odata+offset, self.iv)
+            rc = EVP_EncryptInit_ex(self.ctx, self.cipher(), NULL, NULL, NULL)
+            if not rc:
+                raise CryptoError('EVP_EncryptInit_ex failed')
+            if not EVP_CIPHER_CTX_ctrl(self.ctx, EVP_CTRL_GCM_SET_IVLEN, self.iv_len, NULL):
+                raise CryptoError('EVP_CIPHER_CTX_ctrl SET IVLEN failed')
+            rc = EVP_EncryptInit_ex(self.ctx, NULL, NULL, self.enc_key, self.iv)
+            if not rc:
+                raise CryptoError('EVP_EncryptInit_ex failed')
+            rc = EVP_EncryptUpdate(self.ctx, NULL, &olen, <const unsigned char*> hdata.buf+aoffset, alen)
+            if not rc:
+                raise CryptoError('EVP_EncryptUpdate failed')
+            if not EVP_EncryptUpdate(self.ctx, NULL, &olen, odata+offset, self.iv_len):
+                raise CryptoError('EVP_EncryptUpdate failed')
+            offset += self.iv_len
+            rc = EVP_EncryptUpdate(self.ctx, odata+offset, &olen, <const unsigned char*> idata.buf, ilen)
+            if not rc:
+                raise CryptoError('EVP_EncryptUpdate failed')
+            offset += olen
+            rc = EVP_EncryptFinal_ex(self.ctx, odata+offset, &olen)
+            if not rc:
+                raise CryptoError('EVP_EncryptFinal_ex failed')
+            offset += olen
+            if not EVP_CIPHER_CTX_ctrl(self.ctx, EVP_CTRL_GCM_GET_TAG, self.mac_len, odata+hlen):
+                raise CryptoError('EVP_CIPHER_CTX_ctrl GET TAG failed')
+            self.blocks = block_count
+            return odata[:offset]
+        finally:
+            PyMem_Free(odata)
+            PyBuffer_Release(&hdata)
+            PyBuffer_Release(&idata)
+
+    def decrypt(self, envelope):
+        """
+        authenticate aad + iv + cdata, decrypt cdata, ignore header bytes up to aad_offset.
+        """
+        # AES-GCM, AES-OCB, CHACHA20 ciphers all add a internal 32bit counter to the 96bit (12Byte)
+        # IV we provide, thus we must not decrypt more than 2^32 cipher blocks with same IV):
+        approx_block_count = self.block_count(len(envelope))  # sloppy, but good enough for borg
+        if approx_block_count > 2**32:
+            raise ValueError('too much data, would overflow internal 32bit counter')
+        cdef int ilen = len(envelope)
+        cdef int hlen = self.header_len
+        assert hlen == self.header_len
+        cdef int aoffset = self.aad_offset
+        cdef int alen = hlen - aoffset
+        cdef unsigned char *odata = <unsigned char *>PyMem_Malloc(ilen + self.cipher_blk_len)
+        if not odata:
+            raise MemoryError
+        cdef int olen
+        cdef int offset
+        cdef Py_buffer idata = ro_buffer(envelope)
+        try:
+            if not EVP_DecryptInit_ex(self.ctx, self.cipher(), NULL, NULL, NULL):
+                raise CryptoError('EVP_DecryptInit_ex failed')
+            iv = self.fetch_iv(<unsigned char *> idata.buf+hlen+self.mac_len)
+            self.set_iv(iv)
+            if not EVP_CIPHER_CTX_ctrl(self.ctx, EVP_CTRL_GCM_SET_IVLEN, self.iv_len, NULL):
+                raise CryptoError('EVP_CIPHER_CTX_ctrl SET IVLEN failed')
+            if not EVP_DecryptInit_ex(self.ctx, NULL, NULL, self.enc_key, iv):
+                raise CryptoError('EVP_DecryptInit_ex failed')
+            if not EVP_CIPHER_CTX_ctrl(self.ctx, EVP_CTRL_GCM_SET_TAG, self.mac_len, <void *> idata.buf+hlen):
+                raise CryptoError('EVP_CIPHER_CTX_ctrl SET TAG failed')
+            rc = EVP_DecryptUpdate(self.ctx, NULL, &olen, <const unsigned char*> idata.buf+aoffset, alen)
+            if not rc:
+                raise CryptoError('EVP_DecryptUpdate failed')
+            if not EVP_DecryptUpdate(self.ctx, NULL, &olen,
+                                     <const unsigned char*> idata.buf+hlen+self.mac_len, self.iv_len):
+                raise CryptoError('EVP_DecryptUpdate failed')
+            offset = 0
+            rc = EVP_DecryptUpdate(self.ctx, odata+offset, &olen,
+                                   <const unsigned char*> idata.buf+hlen+self.mac_len+self.iv_len,
+                                   ilen-hlen-self.mac_len-self.iv_len)
+            if not rc:
+                raise CryptoError('EVP_DecryptUpdate failed')
+            offset += olen
+            rc = EVP_DecryptFinal_ex(self.ctx, odata+offset, &olen)
+            if rc <= 0:
+                # a failure here means corrupted or tampered tag (mac) or data.
+                raise IntegrityError('Authentication / EVP_DecryptFinal_ex failed')
+            offset += olen
+            self.blocks = self.block_count(offset)
+            return odata[:offset]
+        finally:
+            PyMem_Free(odata)
+            PyBuffer_Release(&idata)
+
+    def block_count(self, length):
+        return num_cipher_blocks(length, self.cipher_blk_len)
+
+    def set_iv(self, iv):
+        # set_iv needs to be called before each encrypt() call,
+        # because encrypt does a full initialisation of the cipher context.
+        if isinstance(iv, int):
+            iv = iv.to_bytes(self.iv_len, byteorder='big')
+        assert isinstance(iv, bytes) and len(iv) == self.iv_len
+        for i in range(self.iv_len):
+            self.iv[i] = iv[i]
+        self.blocks = 0  # number of cipher blocks encrypted with this IV
+
+    def next_iv(self):
+        # call this after encrypt() to get the next iv (int) for the next encrypt() call
+        # AES-GCM, AES-OCB, CHACHA20 ciphers all add a internal 32bit counter to the 96bit
+        # (12 byte) IV we provide, thus we only need to increment the IV by 1.
+        iv = int.from_bytes(self.iv[:self.iv_len], byteorder='big')
+        return iv + 1
+
+    cdef fetch_iv(self, unsigned char * iv_in):
+        return iv_in[0:self.iv_len]
+
+    cdef store_iv(self, unsigned char * iv_out, unsigned char * iv):
+        cdef int i
+        for i in range(self.iv_len):
+            iv_out[i] = iv[i]
+
+    def extract_iv(self, envelope):
+        offset = self.header_len + self.mac_len
+        return bytes_to_long(envelope[offset:offset+self.iv_len])
+
+
+cdef class _AES_BASE(_AEAD_BASE):
+    def __init__(self, *args, **kwargs):
+        self.cipher_blk_len = 16
+        super().__init__(*args, **kwargs)
+
+
+cdef class _CHACHA_BASE(_AEAD_BASE):
+    def __init__(self, *args, **kwargs):
+        self.cipher_blk_len = 64
+        super().__init__(*args, **kwargs)
+
+
+cdef class AES256_GCM(_AES_BASE):
+    @staticmethod
+    def requirements_check():
+        if OPENSSL_VERSION_NUMBER < 0x10001040:
+            raise ValueError('AES GCM requires OpenSSL >= 1.0.1d. Detected: OpenSSL %08x' % OPENSSL_VERSION_NUMBER)
+
+    def __init__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        self.requirements_check()
+        self.cipher = EVP_aes_256_gcm
+        super().__init__(mac_key, enc_key, iv=iv, header_len=header_len, aad_offset=aad_offset)
+
+
+cdef class AES256_OCB(_AES_BASE):
+    @staticmethod
+    def requirements_check():
+        if OPENSSL_VERSION_NUMBER < 0x10100000:
+            raise ValueError('AES OCB requires OpenSSL >= 1.1.0. Detected: OpenSSL %08x' % OPENSSL_VERSION_NUMBER)
+
+    def __init__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        self.requirements_check()
+        self.cipher = EVP_aes_256_ocb
+        super().__init__(mac_key, enc_key, iv=iv, header_len=header_len, aad_offset=aad_offset)
+
+
+cdef class CHACHA20_POLY1305(_CHACHA_BASE):
+    @staticmethod
+    def requirements_check():
+        if OPENSSL_VERSION_NUMBER < 0x10100000:
+            raise ValueError('CHACHA20-POLY1305 requires OpenSSL >= 1.1.0. Detected: OpenSSL %08x' % OPENSSL_VERSION_NUMBER)
+
+    def __init__(self, mac_key, enc_key, iv=None, header_len=1, aad_offset=1):
+        self.requirements_check()
+        self.cipher = EVP_chacha20_poly1305
+        super().__init__(mac_key, enc_key, iv=iv, header_len=header_len, aad_offset=aad_offset)
+
+
+cdef class AES:
+    """A thin wrapper around the OpenSSL EVP cipher API - for legacy code, like key file encryption"""
+    cdef CIPHER cipher
+    cdef EVP_CIPHER_CTX *ctx
+    cdef unsigned char *enc_key
+    cdef int cipher_blk_len
+    cdef int iv_len
+    cdef unsigned char iv[16]
+    cdef long long blocks
+
+    def __init__(self, enc_key, iv=None):
+        assert isinstance(enc_key, bytes) and len(enc_key) == 32
+        self.enc_key = enc_key
+        self.iv_len = 16
+        assert sizeof(self.iv) == self.iv_len
+        self.cipher = EVP_aes_256_ctr
+        self.cipher_blk_len = 16
+        if iv is not None:
+            self.set_iv(iv)
+        else:
+            self.blocks = -1  # make sure set_iv is called before encrypt
+
+    def __cinit__(self, enc_key, iv=None):
+        self.ctx = EVP_CIPHER_CTX_new()
+
+    def __dealloc__(self):
+        EVP_CIPHER_CTX_free(self.ctx)
+
+    def encrypt(self, data, iv=None):
+        if iv is not None:
+            self.set_iv(iv)
+        assert self.blocks == 0, 'iv needs to be set before encrypt is called'
+        cdef Py_buffer idata = ro_buffer(data)
+        cdef int ilen = len(data)
+        cdef int offset
+        cdef int olen
+        cdef unsigned char *odata = <unsigned char *>PyMem_Malloc(ilen + self.cipher_blk_len)
+        if not odata:
             raise MemoryError
         try:
-            if not EVP_EncryptUpdate(self.ctx, out, &outl, <const unsigned char*> data_buf.buf, inl):
+            if not EVP_EncryptInit_ex(self.ctx, self.cipher(), NULL, self.enc_key, self.iv):
+                raise Exception('EVP_EncryptInit_ex failed')
+            offset = 0
+            if not EVP_EncryptUpdate(self.ctx, odata, &olen, <const unsigned char*> idata.buf, ilen):
                 raise Exception('EVP_EncryptUpdate failed')
-            ctl = outl
-            if not EVP_EncryptFinal_ex(self.ctx, out+ctl, &outl):
+            offset += olen
+            if not EVP_EncryptFinal_ex(self.ctx, odata+offset, &olen):
                 raise Exception('EVP_EncryptFinal failed')
-            ctl += outl
-            self.blocks += num_aes_blocks(ctl)
-            return out[:ctl]
+            offset += olen
+            self.blocks = self.block_count(offset)
+            return odata[:offset]
         finally:
-            free(out)
-            PyBuffer_Release(&data_buf)
+            PyMem_Free(odata)
+            PyBuffer_Release(&idata)
 
     def decrypt(self, data):
-        cdef Py_buffer data_buf = ro_buffer(data)
-        cdef int inl = len(data)
-        cdef int ptl = 0
-        cdef int outl = 0
-        # note: modes that use padding, need up to one extra AES block (16b).
-        # This is what the openssl docs say. I am not sure this is correct,
-        # but OTOH it will not cause any harm if our buffer is a little bigger.
-        cdef unsigned char *out = <unsigned char *>malloc(inl+16)
-        if not out:
+        cdef Py_buffer idata = ro_buffer(data)
+        cdef int ilen = len(data)
+        cdef int offset
+        cdef int olen
+        cdef unsigned char *odata = <unsigned char *>PyMem_Malloc(ilen + self.cipher_blk_len)
+        if not odata:
             raise MemoryError
         try:
-            if not EVP_DecryptUpdate(self.ctx, out, &outl, <const unsigned char*> data_buf.buf, inl):
+            # Set cipher type and mode
+            if not EVP_DecryptInit_ex(self.ctx, self.cipher(), NULL, self.enc_key, self.iv):
+                raise Exception('EVP_DecryptInit_ex failed')
+            offset = 0
+            if not EVP_DecryptUpdate(self.ctx, odata, &olen, <const unsigned char*> idata.buf, ilen):
                 raise Exception('EVP_DecryptUpdate failed')
-            ptl = outl
-            if EVP_DecryptFinal_ex(self.ctx, out+ptl, &outl) <= 0:
+            offset += olen
+            if EVP_DecryptFinal_ex(self.ctx, odata+offset, &olen) <= 0:
                 # this error check is very important for modes with padding or
                 # authentication. for them, a failure here means corrupted data.
                 # CTR mode does not use padding nor authentication.
                 raise Exception('EVP_DecryptFinal failed')
-            ptl += outl
-            self.blocks += num_aes_blocks(inl)
-            return out[:ptl]
+            offset += olen
+            self.blocks = self.block_count(ilen)
+            return odata[:offset]
         finally:
-            free(out)
-            PyBuffer_Release(&data_buf)
+            PyMem_Free(odata)
+            PyBuffer_Release(&idata)
+
+    def block_count(self, length):
+        return num_cipher_blocks(length, self.cipher_blk_len)
+
+    def set_iv(self, iv):
+        # set_iv needs to be called before each encrypt() call,
+        # because encrypt does a full initialisation of the cipher context.
+        if isinstance(iv, int):
+            iv = iv.to_bytes(self.iv_len, byteorder='big')
+        assert isinstance(iv, bytes) and len(iv) == self.iv_len
+        for i in range(self.iv_len):
+            self.iv[i] = iv[i]
+        self.blocks = 0  # number of cipher blocks encrypted with this IV
+
+    def next_iv(self):
+        # call this after encrypt() to get the next iv (int) for the next encrypt() call
+        iv = int.from_bytes(self.iv[:self.iv_len], byteorder='big')
+        return iv + self.blocks
+
 
 
 def hmac_sha256(key, data):
@@ -210,7 +816,7 @@ def hmac_sha256(key, data):
         with nogil:
             rc = HMAC(EVP_sha256(), key_ptr, key_len, <const unsigned char*> data_buf.buf, data_buf.len, md, NULL)
         if rc != md:
-            raise Exception('HMAC(EVP_sha256) failed')
+            raise CryptoError('HMAC(EVP_sha256) failed')
     finally:
         PyBuffer_Release(&data_buf)
     return PyBytes_FromStringAndSize(<char*> &md[0], 32)
diff --git a/src/borg/crypto/nonces.py b/src/borg/crypto/nonces.py
index ec4700acf..39ec3d723 100644
--- a/src/borg/crypto/nonces.py
+++ b/src/borg/crypto/nonces.py
@@ -14,9 +14,8 @@ NONCE_SPACE_RESERVATION = 2**28  # This in units of AES blocksize (16 bytes)
 
 
 class NonceManager:
-    def __init__(self, repository, enc_cipher, manifest_nonce):
+    def __init__(self, repository, manifest_nonce):
         self.repository = repository
-        self.enc_cipher = enc_cipher
         self.end_of_nonce_reservation = None
         self.manifest_nonce = manifest_nonce
         self.nonce_file = os.path.join(get_security_dir(self.repository.id_str), 'nonce')
@@ -47,7 +46,15 @@ class NonceManager:
     def commit_repo_nonce_reservation(self, next_unreserved, start_nonce):
         self.repository.commit_nonce_reservation(next_unreserved, start_nonce)
 
-    def ensure_reservation(self, nonce_space_needed):
+    def ensure_reservation(self, nonce, nonce_space_needed):
+        """
+        Call this before doing encryption, give current, yet unused, integer IV as <nonce>
+        and the amount of subsequent (counter-like) IVs needed as <nonce_space_needed>.
+        Return value is the IV (counter) integer you shall use for encryption.
+
+        Note: this method may return the <nonce> you gave, if a reservation for it exists or
+              can be established, so make sure you give a unused nonce.
+        """
         # Nonces may never repeat, even if a transaction aborts or the system crashes.
         # Therefore a part of the nonce space is reserved before any nonce is used for encryption.
         # As these reservations are committed to permanent storage before any nonce is used, this protects
@@ -64,24 +71,17 @@ class NonceManager:
 
         if self.end_of_nonce_reservation:
             # we already got a reservation, if nonce_space_needed still fits everything is ok
-            next_nonce = int.from_bytes(self.enc_cipher.iv, byteorder='big')
+            next_nonce = nonce
             assert next_nonce <= self.end_of_nonce_reservation
             if next_nonce + nonce_space_needed <= self.end_of_nonce_reservation:
-                return
+                return next_nonce
 
         repo_free_nonce = self.get_repo_free_nonce()
         local_free_nonce = self.get_local_free_nonce()
         free_nonce_space = max(x for x in (repo_free_nonce, local_free_nonce, self.manifest_nonce, self.end_of_nonce_reservation) if x is not None)
         reservation_end = free_nonce_space + nonce_space_needed + NONCE_SPACE_RESERVATION
         assert reservation_end < MAX_REPRESENTABLE_NONCE
-        if self.end_of_nonce_reservation is None:
-            # initialization, reset the encryption cipher to the start of the reservation
-            self.enc_cipher.reset(None, free_nonce_space.to_bytes(16, byteorder='big'))
-        else:
-            # expand existing reservation if possible
-            if free_nonce_space != self.end_of_nonce_reservation:
-                # some other client got an interleaved reservation, skip partial space in old reservation to avoid overlap
-                self.enc_cipher.reset(None, free_nonce_space.to_bytes(16, byteorder='big'))
         self.commit_repo_nonce_reservation(reservation_end, repo_free_nonce)
         self.commit_local_nonce_reservation(reservation_end, local_free_nonce)
         self.end_of_nonce_reservation = reservation_end
+        return free_nonce_space
diff --git a/src/borg/helpers.py b/src/borg/helpers.py
index 1b6d38d83..89246b921 100644
--- a/src/borg/helpers.py
+++ b/src/borg/helpers.py
@@ -91,7 +91,7 @@ class ErrorWithTraceback(Error):
     traceback = True
 
 
-class IntegrityError(ErrorWithTraceback):
+class IntegrityError(ErrorWithTraceback, borg.crypto.low_level.IntegrityError):
     """Data integrity error: {}"""
 
 
diff --git a/src/borg/selftest.py b/src/borg/selftest.py
index d2ea9a769..e8605f7cb 100644
--- a/src/borg/selftest.py
+++ b/src/borg/selftest.py
@@ -30,7 +30,7 @@ SELFTEST_CASES = [
     ChunkerTestCase,
 ]
 
-SELFTEST_COUNT = 35
+SELFTEST_COUNT = 37
 
 
 class SelfTestResult(TestResult):
diff --git a/src/borg/testsuite/archiver.py b/src/borg/testsuite/archiver.py
index 58b52866d..3d2783e1f 100644
--- a/src/borg/testsuite/archiver.py
+++ b/src/borg/testsuite/archiver.py
@@ -36,7 +36,7 @@ from ..archive import Archive, ChunkBuffer, flags_noatime, flags_normal
 from ..archiver import Archiver, parse_storage_quota
 from ..cache import Cache, LocalCache
 from ..constants import *  # NOQA
-from ..crypto.low_level import bytes_to_long, num_aes_blocks
+from ..crypto.low_level import bytes_to_long, num_cipher_blocks
 from ..crypto.key import KeyfileKeyBase, RepoKey, KeyfileKey, Passphrase, TAMRequiredError
 from ..crypto.keymanager import RepoIdMismatch, NotABorgKeyFile
 from ..crypto.file_integrity import FileIntegrityError
@@ -2169,7 +2169,7 @@ class ArchiverTestCase(ArchiverTestCaseBase):
                     hash = sha256(data).digest()
                     if hash not in seen:
                         seen.add(hash)
-                        num_blocks = num_aes_blocks(len(data) - 41)
+                        num_blocks = num_cipher_blocks(len(data) - 41)
                         nonce = bytes_to_long(data[33:41])
                         for counter in range(nonce, nonce + num_blocks):
                             self.assert_not_in(counter, used)
diff --git a/src/borg/testsuite/crypto.py b/src/borg/testsuite/crypto.py
index 6406064d1..a4e822c5d 100644
--- a/src/borg/testsuite/crypto.py
+++ b/src/borg/testsuite/crypto.py
@@ -1,8 +1,10 @@
 from binascii import hexlify, unhexlify
 
-from ..crypto.low_level import AES, bytes_to_long, bytes_to_int, long_to_bytes, hmac_sha256, blake2b_256
-from ..crypto.low_level import increment_iv, bytes16_to_int, int_to_bytes16
+from ..crypto.low_level import AES256_CTR_HMAC_SHA256, AES256_GCM, AES256_OCB, CHACHA20_POLY1305, UNENCRYPTED, \
+                               IntegrityError, blake2b_256, hmac_sha256, openssl10
+from ..crypto.low_level import bytes_to_long, bytes_to_int, long_to_bytes
 from ..crypto.low_level import hkdf_hmac_sha512
+
 from . import BaseTestCase
 
 # Note: these tests are part of the self test, do not use or import py.test functionality here.
@@ -18,42 +20,168 @@ class CryptoTestCase(BaseTestCase):
         self.assert_equal(bytes_to_long(b'\0\0\0\0\0\0\0\1'), 1)
         self.assert_equal(long_to_bytes(1), b'\0\0\0\0\0\0\0\1')
 
-    def test_bytes16_to_int(self):
-        self.assert_equal(bytes16_to_int(b'\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\1'), 1)
-        self.assert_equal(int_to_bytes16(1), b'\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\1')
-        self.assert_equal(bytes16_to_int(b'\0\0\0\0\0\0\0\1\0\0\0\0\0\0\0\0'), 2 ** 64)
-        self.assert_equal(int_to_bytes16(2 ** 64), b'\0\0\0\0\0\0\0\1\0\0\0\0\0\0\0\0')
+    def test_UNENCRYPTED(self):
+        iv = b''  # any IV is ok, it just must be set and not None
+        data = b'data'
+        header = b'header'
+        cs = UNENCRYPTED(None, None, iv, header_len=6)
+        envelope = cs.encrypt(data, header=header)
+        self.assert_equal(envelope, header + data)
+        got_data = cs.decrypt(envelope)
+        self.assert_equal(got_data, data)
 
-    def test_increment_iv(self):
-        iv0 = b'\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0'
-        iv1 = b'\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\1'
-        iv2 = b'\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\2'
-        self.assert_equal(increment_iv(iv0, 0), iv0)
-        self.assert_equal(increment_iv(iv0, 1), iv1)
-        self.assert_equal(increment_iv(iv0, 2), iv2)
-        iva = b'\0\0\0\0\0\0\0\0\xff\xff\xff\xff\xff\xff\xff\xff'
-        ivb = b'\0\0\0\0\0\0\0\1\x00\x00\x00\x00\x00\x00\x00\x00'
-        ivc = b'\0\0\0\0\0\0\0\1\x00\x00\x00\x00\x00\x00\x00\x01'
-        self.assert_equal(increment_iv(iva, 0), iva)
-        self.assert_equal(increment_iv(iva, 1), ivb)
-        self.assert_equal(increment_iv(iva, 2), ivc)
-        self.assert_equal(increment_iv(iv0, 2**64), ivb)
-
-    def test_aes(self):
-        key = b'X' * 32
+    def test_AES256_CTR_HMAC_SHA256(self):
+        # this tests the layout as in attic / borg < 1.2 (1 type byte, no aad)
+        mac_key = b'Y' * 32
+        enc_key = b'X' * 32
+        iv = 0
         data = b'foo' * 10
-        # encrypt
-        aes = AES(is_encrypt=True, key=key)
-        self.assert_equal(bytes_to_long(aes.iv, 8), 0)
-        cdata = aes.encrypt(data)
+        header = b'\x42'
+        # encrypt-then-mac
+        cs = AES256_CTR_HMAC_SHA256(mac_key, enc_key, iv, header_len=1, aad_offset=1)
+        hdr_mac_iv_cdata = cs.encrypt(data, header=header)
+        hdr = hdr_mac_iv_cdata[0:1]
+        mac = hdr_mac_iv_cdata[1:33]
+        iv = hdr_mac_iv_cdata[33:41]
+        cdata = hdr_mac_iv_cdata[41:]
+        self.assert_equal(hexlify(hdr), b'42')
+        self.assert_equal(hexlify(mac), b'af90b488b0cc4a8f768fe2d6814fa65aec66b148135e54f7d4d29a27f22f57a8')
+        self.assert_equal(hexlify(iv), b'0000000000000000')
         self.assert_equal(hexlify(cdata), b'c6efb702de12498f34a2c2bbc8149e759996d08bf6dc5c610aefc0c3a466')
-        self.assert_equal(bytes_to_long(aes.iv, 8), 2)
-        # decrypt
-        aes = AES(is_encrypt=False, key=key)
-        self.assert_equal(bytes_to_long(aes.iv, 8), 0)
-        pdata = aes.decrypt(cdata)
+        self.assert_equal(cs.next_iv(), 2)
+        # auth-then-decrypt
+        cs = AES256_CTR_HMAC_SHA256(mac_key, enc_key, header_len=len(header), aad_offset=1)
+        pdata = cs.decrypt(hdr_mac_iv_cdata)
         self.assert_equal(data, pdata)
-        self.assert_equal(bytes_to_long(aes.iv, 8), 2)
+        self.assert_equal(cs.next_iv(), 2)
+        # auth-failure due to corruption (corrupted data)
+        cs = AES256_CTR_HMAC_SHA256(mac_key, enc_key, header_len=len(header), aad_offset=1)
+        hdr_mac_iv_cdata_corrupted = hdr_mac_iv_cdata[:41] + b'\0' + hdr_mac_iv_cdata[42:]
+        self.assert_raises(IntegrityError,
+                           lambda: cs.decrypt(hdr_mac_iv_cdata_corrupted))
+
+    def test_AES256_CTR_HMAC_SHA256_aad(self):
+        mac_key = b'Y' * 32
+        enc_key = b'X' * 32
+        iv = 0
+        data = b'foo' * 10
+        header = b'\x12\x34\x56'
+        # encrypt-then-mac
+        cs = AES256_CTR_HMAC_SHA256(mac_key, enc_key, iv, header_len=3, aad_offset=1)
+        hdr_mac_iv_cdata = cs.encrypt(data, header=header)
+        hdr = hdr_mac_iv_cdata[0:3]
+        mac = hdr_mac_iv_cdata[3:35]
+        iv = hdr_mac_iv_cdata[35:43]
+        cdata = hdr_mac_iv_cdata[43:]
+        self.assert_equal(hexlify(hdr), b'123456')
+        self.assert_equal(hexlify(mac), b'7659a915d9927072ef130258052351a17ef882692893c3850dd798c03d2dd138')
+        self.assert_equal(hexlify(iv), b'0000000000000000')
+        self.assert_equal(hexlify(cdata), b'c6efb702de12498f34a2c2bbc8149e759996d08bf6dc5c610aefc0c3a466')
+        self.assert_equal(cs.next_iv(), 2)
+        # auth-then-decrypt
+        cs = AES256_CTR_HMAC_SHA256(mac_key, enc_key, header_len=len(header), aad_offset=1)
+        pdata = cs.decrypt(hdr_mac_iv_cdata)
+        self.assert_equal(data, pdata)
+        self.assert_equal(cs.next_iv(), 2)
+        # auth-failure due to corruption (corrupted aad)
+        cs = AES256_CTR_HMAC_SHA256(mac_key, enc_key, header_len=len(header), aad_offset=1)
+        hdr_mac_iv_cdata_corrupted = hdr_mac_iv_cdata[:1] + b'\0' + hdr_mac_iv_cdata[2:]
+        self.assert_raises(IntegrityError,
+                           lambda: cs.decrypt(hdr_mac_iv_cdata_corrupted))
+
+    def test_AE(self):
+        # used in legacy-like layout (1 type byte, no aad)
+        mac_key = None
+        enc_key = b'X' * 32
+        iv = 0
+        data = b'foo' * 10
+        header = b'\x23'
+        tests = [
+            # ciphersuite class, exp_mac, exp_cdata
+            (AES256_GCM,
+             b'66a438843aa41a087d6a7ed1dc1f3c4c',
+             b'5bbb40be14e4bcbfc75715b77b1242d590d2bf9f7f8a8a910b4469888689', )
+        ]
+        if not openssl10:
+            tests += [
+                (AES256_OCB,
+                 b'b6909c23c9aaebd9abbe1ff42097652d',
+                 b'877ce46d2f62dee54699cebc3ba41d9ab613f7c486778c1b3636664b1493', ),
+                (CHACHA20_POLY1305,
+                 b'fd08594796e0706cde1e8b461e3e0555',
+                 b'a093e4b0387526f085d3c40cca84a35230a5c0dd766453b77ba38bcff775', )
+            ]
+        for cs_cls, exp_mac, exp_cdata in tests:
+            # print(repr(cs_cls))
+            # encrypt/mac
+            cs = cs_cls(mac_key, enc_key, iv, header_len=1, aad_offset=1)
+            hdr_mac_iv_cdata = cs.encrypt(data, header=header)
+            hdr = hdr_mac_iv_cdata[0:1]
+            mac = hdr_mac_iv_cdata[1:17]
+            iv = hdr_mac_iv_cdata[17:29]
+            cdata = hdr_mac_iv_cdata[29:]
+            self.assert_equal(hexlify(hdr), b'23')
+            self.assert_equal(hexlify(mac), exp_mac)
+            self.assert_equal(hexlify(iv), b'000000000000000000000000')
+            self.assert_equal(hexlify(cdata), exp_cdata)
+            self.assert_equal(cs.next_iv(), 1)
+            # auth/decrypt
+            cs = cs_cls(mac_key, enc_key, header_len=len(header), aad_offset=1)
+            pdata = cs.decrypt(hdr_mac_iv_cdata)
+            self.assert_equal(data, pdata)
+            self.assert_equal(cs.next_iv(), 1)
+            # auth-failure due to corruption (corrupted data)
+            cs = cs_cls(mac_key, enc_key, header_len=len(header), aad_offset=1)
+            hdr_mac_iv_cdata_corrupted = hdr_mac_iv_cdata[:29] + b'\0' + hdr_mac_iv_cdata[30:]
+            self.assert_raises(IntegrityError,
+                               lambda: cs.decrypt(hdr_mac_iv_cdata_corrupted))
+
+    def test_AEAD(self):
+        # test with aad
+        mac_key = None
+        enc_key = b'X' * 32
+        iv = 0
+        data = b'foo' * 10
+        header = b'\x12\x34\x56'
+        tests = [
+            # ciphersuite class, exp_mac, exp_cdata
+            (AES256_GCM,
+             b'4fb0e5b0a0bca57527352cc6240e7cca',
+             b'5bbb40be14e4bcbfc75715b77b1242d590d2bf9f7f8a8a910b4469888689', )
+        ]
+        if not openssl10:
+            tests += [
+                (AES256_OCB,
+                 b'f2748c412af1c7ead81863a18c2c1893',
+                 b'877ce46d2f62dee54699cebc3ba41d9ab613f7c486778c1b3636664b1493', ),
+                (CHACHA20_POLY1305,
+                 b'b7e7c9a79f2404e14f9aad156bf091dd',
+                 b'a093e4b0387526f085d3c40cca84a35230a5c0dd766453b77ba38bcff775', )
+            ]
+        for cs_cls, exp_mac, exp_cdata in tests:
+            # print(repr(cs_cls))
+            # encrypt/mac
+            cs = cs_cls(mac_key, enc_key, iv, header_len=3, aad_offset=1)
+            hdr_mac_iv_cdata = cs.encrypt(data, header=header)
+            hdr = hdr_mac_iv_cdata[0:3]
+            mac = hdr_mac_iv_cdata[3:19]
+            iv = hdr_mac_iv_cdata[19:31]
+            cdata = hdr_mac_iv_cdata[31:]
+            self.assert_equal(hexlify(hdr), b'123456')
+            self.assert_equal(hexlify(mac), exp_mac)
+            self.assert_equal(hexlify(iv), b'000000000000000000000000')
+            self.assert_equal(hexlify(cdata), exp_cdata)
+            self.assert_equal(cs.next_iv(), 1)
+            # auth/decrypt
+            cs = cs_cls(mac_key, enc_key, header_len=len(header), aad_offset=1)
+            pdata = cs.decrypt(hdr_mac_iv_cdata)
+            self.assert_equal(data, pdata)
+            self.assert_equal(cs.next_iv(), 1)
+            # auth-failure due to corruption (corrupted aad)
+            cs = cs_cls(mac_key, enc_key, header_len=len(header), aad_offset=1)
+            hdr_mac_iv_cdata_corrupted = hdr_mac_iv_cdata[:1] + b'\0' + hdr_mac_iv_cdata[2:]
+            self.assert_raises(IntegrityError,
+                               lambda: cs.decrypt(hdr_mac_iv_cdata_corrupted))
 
     def test_hmac_sha256(self):
         # RFC 4231 test vectors
diff --git a/src/borg/testsuite/key.py b/src/borg/testsuite/key.py
index 6a7a6c8d7..075311744 100644
--- a/src/borg/testsuite/key.py
+++ b/src/borg/testsuite/key.py
@@ -13,7 +13,8 @@ from ..crypto.key import PlaintextKey, PassphraseKey, AuthenticatedKey, RepoKey,
 from ..crypto.key import ID_HMAC_SHA_256, ID_BLAKE2b_256
 from ..crypto.key import TAMRequiredError, TAMInvalid, TAMUnsupportedSuiteError, UnsupportedManifestError
 from ..crypto.key import identify_key
-from ..crypto.low_level import bytes_to_long, num_aes_blocks
+from ..crypto.low_level import bytes_to_long
+from ..crypto.low_level import IntegrityError as IntegrityErrorBase
 from ..helpers import IntegrityError
 from ..helpers import Location
 from ..helpers import StableDict
@@ -75,6 +76,7 @@ class TestKey:
         AuthenticatedKey,
         KeyfileKey,
         RepoKey,
+        AuthenticatedKey,
         Blake2KeyfileKey,
         Blake2RepoKey,
         Blake2AuthenticatedKey,
@@ -115,16 +117,16 @@ class TestKey:
     def test_keyfile(self, monkeypatch, keys_dir):
         monkeypatch.setenv('BORG_PASSPHRASE', 'test')
         key = KeyfileKey.create(self.MockRepository(), self.MockArgs())
-        assert bytes_to_long(key.enc_cipher.iv, 8) == 0
+        assert key.cipher.next_iv() == 0
         manifest = key.encrypt(b'ABC')
-        assert key.extract_nonce(manifest) == 0
+        assert key.cipher.extract_iv(manifest) == 0
         manifest2 = key.encrypt(b'ABC')
         assert manifest != manifest2
         assert key.decrypt(None, manifest) == key.decrypt(None, manifest2)
-        assert key.extract_nonce(manifest2) == 1
-        iv = key.extract_nonce(manifest)
+        assert key.cipher.extract_iv(manifest2) == 1
+        iv = key.cipher.extract_iv(manifest)
         key2 = KeyfileKey.detect(self.MockRepository(), manifest)
-        assert bytes_to_long(key2.enc_cipher.iv, 8) >= iv + num_aes_blocks(len(manifest) - KeyfileKey.PAYLOAD_OVERHEAD)
+        assert key2.cipher.next_iv() >= iv + key2.cipher.block_count(len(manifest) - KeyfileKey.PAYLOAD_OVERHEAD)
         # Key data sanity check
         assert len({key2.id_key, key2.enc_key, key2.enc_hmac_key}) == 3
         assert key2.chunk_seed != 0
@@ -138,7 +140,7 @@ class TestKey:
             fd.write("0000000000002000")
         key = KeyfileKey.create(repository, self.MockArgs())
         data = key.encrypt(b'ABC')
-        assert key.extract_nonce(data) == 0x2000
+        assert key.cipher.extract_iv(data) == 0x2000
         assert key.decrypt(None, data) == b'ABC'
 
     def test_keyfile_kfenv(self, tmpdir, monkeypatch):
@@ -183,20 +185,20 @@ class TestKey:
     def test_passphrase(self, keys_dir, monkeypatch):
         monkeypatch.setenv('BORG_PASSPHRASE', 'test')
         key = PassphraseKey.create(self.MockRepository(), None)
-        assert bytes_to_long(key.enc_cipher.iv, 8) == 0
+        assert key.cipher.next_iv() == 0
         assert hexlify(key.id_key) == b'793b0717f9d8fb01c751a487e9b827897ceea62409870600013fbc6b4d8d7ca6'
         assert hexlify(key.enc_hmac_key) == b'b885a05d329a086627412a6142aaeb9f6c54ab7950f996dd65587251f6bc0901'
         assert hexlify(key.enc_key) == b'2ff3654c6daf7381dbbe718d2b20b4f1ea1e34caa6cc65f6bb3ac376b93fed2a'
         assert key.chunk_seed == -775740477
         manifest = key.encrypt(b'ABC')
-        assert key.extract_nonce(manifest) == 0
+        assert key.cipher.extract_iv(manifest) == 0
         manifest2 = key.encrypt(b'ABC')
         assert manifest != manifest2
         assert key.decrypt(None, manifest) == key.decrypt(None, manifest2)
-        assert key.extract_nonce(manifest2) == 1
-        iv = key.extract_nonce(manifest)
+        assert key.cipher.extract_iv(manifest2) == 1
+        iv = key.cipher.extract_iv(manifest)
         key2 = PassphraseKey.detect(self.MockRepository(), manifest)
-        assert bytes_to_long(key2.enc_cipher.iv, 8) == iv + num_aes_blocks(len(manifest) - PassphraseKey.PAYLOAD_OVERHEAD)
+        assert key2.cipher.next_iv() == iv + key2.cipher.block_count(len(manifest))
         assert key.id_key == key2.id_key
         assert key.enc_hmac_key == key2.enc_hmac_key
         assert key.enc_key == key2.enc_key
@@ -208,7 +210,7 @@ class TestKey:
     def _corrupt_byte(self, key, data, offset):
         data = bytearray(data)
         data[offset] ^= 1
-        with pytest.raises(IntegrityError):
+        with pytest.raises(IntegrityErrorBase):
             key.decrypt(b'', data)
 
     def test_decrypt_integrity(self, monkeypatch, keys_dir):
diff --git a/src/borg/testsuite/nonces.py b/src/borg/testsuite/nonces.py
index bfdc3cc7d..d0bc85eaf 100644
--- a/src/borg/testsuite/nonces.py
+++ b/src/borg/testsuite/nonces.py
@@ -33,29 +33,6 @@ class TestNonceManager:
         def commit_nonce_reservation(self, next_unreserved, start_nonce):
             pytest.fail("commit_nonce_reservation should never be called on an old repository")
 
-    class MockEncCipher:
-        def __init__(self, iv):
-            self.iv_set = False  # placeholder, this is never a valid iv
-            self.iv = iv
-
-        def reset(self, key, iv):
-            assert key is None
-            assert iv is not False
-            self.iv_set = iv
-            self.iv = iv
-
-        def expect_iv_and_advance(self, expected_iv, advance):
-            expected_iv = expected_iv.to_bytes(16, byteorder='big')
-            iv_set = self.iv_set
-            assert iv_set == expected_iv
-            self.iv_set = False
-            self.iv = advance.to_bytes(16, byteorder='big')
-
-        def expect_no_reset_and_advance(self, advance):
-            iv_set = self.iv_set
-            assert iv_set is False
-            self.iv = advance.to_bytes(16, byteorder='big')
-
     def setUp(self):
         self.repository = None
 
@@ -70,74 +47,70 @@ class TestNonceManager:
     def test_empty_cache_and_old_server(self, monkeypatch):
         monkeypatch.setattr(nonces, 'NONCE_SPACE_RESERVATION', 0x20)
 
-        enc_cipher = self.MockEncCipher(0x2000)
         self.repository = self.MockOldRepository()
-        manager = NonceManager(self.repository, enc_cipher, 0x2000)
-        manager.ensure_reservation(19)
-        enc_cipher.expect_iv_and_advance(0x2000, 0x2013)
+        manager = NonceManager(self.repository, 0x2000)
+        next_nonce = manager.ensure_reservation(0x2000, 19)
+        assert next_nonce == 0x2000
 
         assert self.cache_nonce() == "0000000000002033"
 
     def test_empty_cache(self, monkeypatch):
         monkeypatch.setattr(nonces, 'NONCE_SPACE_RESERVATION', 0x20)
 
-        enc_cipher = self.MockEncCipher(0x2000)
         self.repository = self.MockRepository()
         self.repository.next_free = 0x2000
-        manager = NonceManager(self.repository, enc_cipher, 0x2000)
-        manager.ensure_reservation(19)
-        enc_cipher.expect_iv_and_advance(0x2000, 0x2013)
+        manager = NonceManager(self.repository, 0x2000)
+        next_nonce = manager.ensure_reservation(0x2000, 19)
+        assert next_nonce == 0x2000
 
         assert self.cache_nonce() == "0000000000002033"
 
     def test_empty_nonce(self, monkeypatch):
         monkeypatch.setattr(nonces, 'NONCE_SPACE_RESERVATION', 0x20)
 
-        enc_cipher = self.MockEncCipher(0x2000)
         self.repository = self.MockRepository()
         self.repository.next_free = None
-        manager = NonceManager(self.repository, enc_cipher, 0x2000)
-        manager.ensure_reservation(19)
-        enc_cipher.expect_iv_and_advance(0x2000, 0x2000 + 19)
+        manager = NonceManager(self.repository, 0x2000)
+        next_nonce = manager.ensure_reservation(0x2000, 19)
+        assert next_nonce == 0x2000
 
         assert self.cache_nonce() == "0000000000002033"
         assert self.repository.next_free == 0x2033
 
         # enough space in reservation
-        manager.ensure_reservation(13)
-        enc_cipher.expect_no_reset_and_advance(0x2000 + 19 + 13)
+        next_nonce = manager.ensure_reservation(0x2013, 13)
+        assert next_nonce == 0x2013
         assert self.cache_nonce() == "0000000000002033"
         assert self.repository.next_free == 0x2033
 
         # just barely enough space in reservation
-        manager.ensure_reservation(19)
-        enc_cipher.expect_no_reset_and_advance(0x2000 + 19 + 13 + 19)
+        next_nonce = manager.ensure_reservation(0x2020, 19)
+        assert next_nonce == 0x2020
         assert self.cache_nonce() == "0000000000002033"
         assert self.repository.next_free == 0x2033
 
         # no space in reservation
-        manager.ensure_reservation(16)
-        enc_cipher.expect_no_reset_and_advance(0x2000 + 19 + 13 + 19 + 16)
+        next_nonce = manager.ensure_reservation(0x2033, 16)
+        assert next_nonce == 0x2033
         assert self.cache_nonce() == "0000000000002063"
         assert self.repository.next_free == 0x2063
 
         # spans reservation boundary
-        manager.ensure_reservation(64)
-        enc_cipher.expect_no_reset_and_advance(0x2000 + 19 + 13 + 19 + 16 + 64)
+        next_nonce = manager.ensure_reservation(0x2043, 64)
+        assert next_nonce == 0x2063
         assert self.cache_nonce() == "00000000000020c3"
         assert self.repository.next_free == 0x20c3
 
     def test_sync_nonce(self, monkeypatch):
         monkeypatch.setattr(nonces, 'NONCE_SPACE_RESERVATION', 0x20)
 
-        enc_cipher = self.MockEncCipher(0x2000)
         self.repository = self.MockRepository()
         self.repository.next_free = 0x2000
         self.set_cache_nonce("0000000000002000")
 
-        manager = NonceManager(self.repository, enc_cipher, 0x2000)
-        manager.ensure_reservation(19)
-        enc_cipher.expect_iv_and_advance(0x2000, 0x2000 + 19)
+        manager = NonceManager(self.repository, 0x2000)
+        next_nonce = manager.ensure_reservation(0x2000, 19)
+        assert next_nonce == 0x2000
 
         assert self.cache_nonce() == "0000000000002033"
         assert self.repository.next_free == 0x2033
@@ -145,14 +118,13 @@ class TestNonceManager:
     def test_server_just_upgraded(self, monkeypatch):
         monkeypatch.setattr(nonces, 'NONCE_SPACE_RESERVATION', 0x20)
 
-        enc_cipher = self.MockEncCipher(0x2000)
         self.repository = self.MockRepository()
         self.repository.next_free = None
         self.set_cache_nonce("0000000000002000")
 
-        manager = NonceManager(self.repository, enc_cipher, 0x2000)
-        manager.ensure_reservation(19)
-        enc_cipher.expect_iv_and_advance(0x2000, 0x2000 + 19)
+        manager = NonceManager(self.repository, 0x2000)
+        next_nonce = manager.ensure_reservation(0x2000, 19)
+        assert next_nonce == 0x2000
 
         assert self.cache_nonce() == "0000000000002033"
         assert self.repository.next_free == 0x2033
@@ -160,13 +132,12 @@ class TestNonceManager:
     def test_transaction_abort_no_cache(self, monkeypatch):
         monkeypatch.setattr(nonces, 'NONCE_SPACE_RESERVATION', 0x20)
 
-        enc_cipher = self.MockEncCipher(0x1000)
         self.repository = self.MockRepository()
         self.repository.next_free = 0x2000
 
-        manager = NonceManager(self.repository, enc_cipher, 0x2000)
-        manager.ensure_reservation(19)
-        enc_cipher.expect_iv_and_advance(0x2000, 0x2000 + 19)
+        manager = NonceManager(self.repository, 0x2000)
+        next_nonce = manager.ensure_reservation(0x1000, 19)
+        assert next_nonce == 0x2000
 
         assert self.cache_nonce() == "0000000000002033"
         assert self.repository.next_free == 0x2033
@@ -174,27 +145,25 @@ class TestNonceManager:
     def test_transaction_abort_old_server(self, monkeypatch):
         monkeypatch.setattr(nonces, 'NONCE_SPACE_RESERVATION', 0x20)
 
-        enc_cipher = self.MockEncCipher(0x1000)
         self.repository = self.MockOldRepository()
         self.set_cache_nonce("0000000000002000")
 
-        manager = NonceManager(self.repository, enc_cipher, 0x2000)
-        manager.ensure_reservation(19)
-        enc_cipher.expect_iv_and_advance(0x2000, 0x2000 + 19)
+        manager = NonceManager(self.repository, 0x2000)
+        next_nonce = manager.ensure_reservation(0x1000, 19)
+        assert next_nonce == 0x2000
 
         assert self.cache_nonce() == "0000000000002033"
 
     def test_transaction_abort_on_other_client(self, monkeypatch):
         monkeypatch.setattr(nonces, 'NONCE_SPACE_RESERVATION', 0x20)
 
-        enc_cipher = self.MockEncCipher(0x1000)
         self.repository = self.MockRepository()
         self.repository.next_free = 0x2000
         self.set_cache_nonce("0000000000001000")
 
-        manager = NonceManager(self.repository, enc_cipher, 0x2000)
-        manager.ensure_reservation(19)
-        enc_cipher.expect_iv_and_advance(0x2000, 0x2000 + 19)
+        manager = NonceManager(self.repository, 0x2000)
+        next_nonce = manager.ensure_reservation(0x1000, 19)
+        assert next_nonce == 0x2000
 
         assert self.cache_nonce() == "0000000000002033"
         assert self.repository.next_free == 0x2033
@@ -202,14 +171,13 @@ class TestNonceManager:
     def test_interleaved(self, monkeypatch):
         monkeypatch.setattr(nonces, 'NONCE_SPACE_RESERVATION', 0x20)
 
-        enc_cipher = self.MockEncCipher(0x2000)
         self.repository = self.MockRepository()
         self.repository.next_free = 0x2000
         self.set_cache_nonce("0000000000002000")
 
-        manager = NonceManager(self.repository, enc_cipher, 0x2000)
-        manager.ensure_reservation(19)
-        enc_cipher.expect_iv_and_advance(0x2000, 0x2000 + 19)
+        manager = NonceManager(self.repository, 0x2000)
+        next_nonce = manager.ensure_reservation(0x2000, 19)
+        assert next_nonce == 0x2000
 
         assert self.cache_nonce() == "0000000000002033"
         assert self.repository.next_free == 0x2033
@@ -218,13 +186,13 @@ class TestNonceManager:
         self.repository.next_free = 0x4000
 
         # enough space in reservation
-        manager.ensure_reservation(12)
-        enc_cipher.expect_no_reset_and_advance(0x2000 + 19 + 12)
+        next_nonce = manager.ensure_reservation(0x2013, 12)
+        assert next_nonce == 0x2013
         assert self.cache_nonce() == "0000000000002033"
         assert self.repository.next_free == 0x4000
 
         # spans reservation boundary
-        manager.ensure_reservation(21)
-        enc_cipher.expect_iv_and_advance(0x4000, 0x4000 + 21)
+        next_nonce = manager.ensure_reservation(0x201f, 21)
+        assert next_nonce == 0x4000
         assert self.cache_nonce() == "0000000000004035"
         assert self.repository.next_free == 0x4035