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Use AAD to validate last chunk and detect file truncation even on a chunk boundary

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This commit is contained in:
Travis Burtrum 2020-01-02 20:35:22 -05:00
parent ec15b8166a
commit 74119989f7
2 changed files with 118 additions and 20 deletions
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19
README.md
View File

@ -15,11 +15,20 @@ pegh -e SUPER_SECRET_1942 <file.txt >file.txt.pegh
# decrypt file.txt.pegh to file.txt with password SUPER_SECRET_1942
pegh -d SUPER_SECRET_1942 <file.txt.pegh >file.txt
# make enrypted backup
# encrypt file.txt to file.txt.pegh with password from password.txt
pegh -f password.txt -i file.txt -o file.txt.pegh
# make encrypted backup
tar czv -C /path/to/dir/ . | pegh SUPER_SECRET_1942 -o foo.tar.gz.pegh
# extract encrypted backup
# extract encrypted backup, the "I'm ok with truncated data" way
pegh SUPER_SECRET_1942 -d -i foo.tar.gz.pegh | tar xzv
# safely extract only complete encrypted backup, the "I have more space than time" way
pegh SUPER_SECRET_1942 -d -i foo.tar.gz.pegh -o foo.tar.gz && tar xzvf foo.tar.gz; rm -f foo.tar.gz
# safely extract only complete encrypted backup, the "I have more time than space" way
pegh SUPER_SECRET_1942 -d -i foo.tar.gz.pegh >/dev/null && pegh SUPER_SECRET_1942 -d -i foo.tar.gz.pegh | tar xzv
```
The easiest way to scale cost/time it takes for bruteforcing is simply to continue doubling -s, on both encryption and decryption commands.
@ -69,7 +78,9 @@ For additional info on scrypt params refer to:
Security
--------
Each chunk is fully decrypted and authenticated in memory before being written out as plaintext, so an attacker may be able to truncate a file, but NEVER flip any bytes or corrupt it. Order is enforced by the incrementing the IV, so re-ordered chunks would be decrypted with the wrong IV and would fail authentication.
Each chunk is fully decrypted and authenticated in memory before being written out as plaintext, so an attacker may be able to truncate a file, but NEVER flip any bytes or corrupt it. Order is enforced by the incrementing the IV, so re-ordered chunks would be decrypted with the wrong IV and would fail authentication. The last chunk is flagged with AAD so we can detect files truncated even on chunk boundaries.
So only fully correct chunks will be output, and only ever in the correct order, but if later chunks are corrupted or the file is truncated, pegh will exit with an error code, and then it's the responsibility of the user/consuming application to do the proper thing with the truncated file it recieved.
Of course standard password bruteforcing is possible, but can be mitigated with increased scrypt parameters and longer password lengths.
@ -78,7 +89,7 @@ pegh file format
pegh implements a simple versioned file format so encryption parameters can change in the future. Numbers here are inclusive 0-based byte array indices, 0th byte is always version number, everything else depends on version number, currently versions 0 and 1 exist.
Version 0, scrypt key derivation, AES-256-GCM encryption, 43 byte header, 16 byte auth tag per chunk. The 12-byte IV for the first chunk is 0, and is incremented by 1 for each successive chunk, if it ever rolls back over to 0 encryption should be aborted (chunk size should be increased).
Version 0, scrypt key derivation, AES-256-GCM encryption, 43 byte header, 16 byte auth tag per chunk. The 12-byte IV for the first chunk is 0, and is incremented by 1 for each successive chunk, if it ever rolls back over to 0 encryption should be aborted (chunk size should be increased). The last chunk has Additional Authenticated Data (AAD) sent in, a single byte value 0, which is used to flag the last chunk and detect file truncation.
| indices | format | value interpretation |
|--------------|---------------------------------------------|-----------------------------------------|

119
pegh.c
View File

@ -73,7 +73,9 @@ const size_t MINIMUM_PASSWORD_LEN = 12;
* |------------------------------------------------------------------------------------------------------|
* | Version 0, scrypt key derivation, AES-256-GCM encryption, 43 byte header, 16 byte auth tag per chunk |
* | The 12-byte IV for the first chunk is 0, and is incremented by 1 for each successive chunk, if it |
* | ever rolls back over to 0 encryption should be aborted (chunk size should be increased). |
* | ever rolls back over to 0 encryption should be aborted (chunk size should be increased). The last |
* | chunk has Additional Authenticated Data (AAD) sent in, a single byte value 0, which is used to flag |
* | the last chunk and detect file truncation. |
* |--------------|---------------------------------------------|-----------------------------------------|
* | indices | format | value interpretation |
* |--------------|---------------------------------------------|-----------------------------------------|
@ -108,6 +110,7 @@ const size_t AEAD_TAG_LEN = 16;
/* libsodium only supports AES on specific platforms, this jazz is to fallback to openssl impls in those cases */
typedef int (*aead_func)(const unsigned char *, const size_t,
const unsigned char *, const unsigned char *,
const unsigned char*, const size_t,
unsigned char *, unsigned char *
);
@ -141,6 +144,7 @@ static const size_t CHUNK_SIZE_MAX_CHACHA = INT_MAX;
int aead_encrypt_openssl(const EVP_CIPHER *type, const unsigned char *plaintext, const size_t plaintext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *ciphertext,
unsigned char *tag
)
@ -166,6 +170,15 @@ int aead_encrypt_openssl(const EVP_CIPHER *type, const unsigned char *plaintext,
if(1 != EVP_EncryptInit_ex(ctx, NULL, NULL, key, iv))
break;
if(aad_len > 0) {
/*
* Provide any AAD data. This can be called zero or more times as
* required
*/
if(1 != EVP_EncryptUpdate(ctx, NULL, &ciphertext_written, aad, (int) aad_len))
break;
}
/*
* Provide the message to be encrypted, and obtain the encrypted output.
* EVP_EncryptUpdate can be called multiple times if necessary
@ -215,6 +228,7 @@ int aead_encrypt_openssl(const EVP_CIPHER *type, const unsigned char *plaintext,
int aead_decrypt_openssl(const EVP_CIPHER *type, const unsigned char *ciphertext, const size_t ciphertext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *tag,
unsigned char *plaintext
)
@ -240,6 +254,15 @@ int aead_decrypt_openssl(const EVP_CIPHER *type, const unsigned char *ciphertext
if(!EVP_DecryptInit_ex(ctx, NULL, NULL, key, iv))
break;
if(aad_len > 0) {
/*
* Provide any AAD data. This can be called zero or more times as
* required
*/
if(1 != EVP_DecryptUpdate(ctx, NULL, &plaintext_written, aad, (int) aad_len))
break;
}
/*
* Provide the message to be decrypted, and obtain the plaintext output.
* EVP_DecryptUpdate can be called multiple times if necessary
@ -276,23 +299,28 @@ int aead_decrypt_openssl(const EVP_CIPHER *type, const unsigned char *ciphertext
int gcm_encrypt_openssl(const unsigned char *plaintext, const size_t plaintext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *ciphertext,
unsigned char *tag
) {
return aead_encrypt_openssl(EVP_aes_256_gcm(), plaintext, plaintext_len,
key, iv,
aad, aad_len,
ciphertext, tag);
}
int gcm_decrypt_openssl(const unsigned char *ciphertext, const size_t ciphertext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *tag,
unsigned char *plaintext
)
{
return aead_decrypt_openssl(EVP_aes_256_gcm(), ciphertext, ciphertext_len,
key, iv, tag,
key, iv,
aad, aad_len,
tag,
plaintext);
}
@ -303,23 +331,28 @@ int gcm_decrypt_openssl(const unsigned char *ciphertext, const size_t ciphertext
int chacha_encrypt(const unsigned char *plaintext, const size_t plaintext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *ciphertext,
unsigned char *tag
) {
return aead_encrypt_openssl(EVP_chacha20_poly1305(), plaintext, plaintext_len,
key, iv,
aad, aad_len,
ciphertext, tag);
}
int chacha_decrypt(const unsigned char *ciphertext, const size_t ciphertext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *tag,
unsigned char *plaintext
)
{
return aead_decrypt_openssl(EVP_chacha20_poly1305(), ciphertext, ciphertext_len,
key, iv, tag,
key, iv,
aad, aad_len,
tag,
plaintext);
}
@ -389,6 +422,7 @@ static const size_t CHUNK_SIZE_MAX_CHACHA = 274877906880; /* 64*(2^32)-64 or sli
int gcm_encrypt_libsodium(const unsigned char *plaintext, const size_t plaintext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *ciphertext,
unsigned char *tag
)
@ -396,7 +430,7 @@ int gcm_encrypt_libsodium(const unsigned char *plaintext, const size_t plaintext
crypto_aead_aes256gcm_encrypt_detached(ciphertext,
tag, NULL,
plaintext, plaintext_len,
NULL, 0,
aad, aad_len,
NULL, iv, key);
return 1;
}
@ -417,6 +451,7 @@ int gcm_encrypt_libsodium(const unsigned char *plaintext, const size_t plaintext
int gcm_decrypt_libsodium(const unsigned char *ciphertext, const size_t ciphertext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *tag,
unsigned char *plaintext
)
@ -425,7 +460,7 @@ int gcm_decrypt_libsodium(const unsigned char *ciphertext, const size_t cipherte
NULL,
ciphertext, (size_t) ciphertext_len,
tag,
NULL, 0,
aad, aad_len,
iv, key) != 0 ? 0 : 1;
}
@ -445,6 +480,7 @@ int gcm_decrypt_libsodium(const unsigned char *ciphertext, const size_t cipherte
int chacha_encrypt(const unsigned char *plaintext, const size_t plaintext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *ciphertext,
unsigned char *tag
)
@ -452,7 +488,7 @@ int chacha_encrypt(const unsigned char *plaintext, const size_t plaintext_len,
crypto_aead_chacha20poly1305_ietf_encrypt_detached(ciphertext,
tag, NULL,
plaintext, plaintext_len,
NULL, 0,
aad, aad_len,
NULL, iv, key);
return 1;
}
@ -473,6 +509,7 @@ int chacha_encrypt(const unsigned char *plaintext, const size_t plaintext_len,
int chacha_decrypt(const unsigned char *ciphertext, const size_t ciphertext_len,
const unsigned char *key,
const unsigned char *iv,
const unsigned char* aad, const size_t aad_len,
unsigned char *tag,
unsigned char *plaintext
)
@ -481,7 +518,7 @@ int chacha_decrypt(const unsigned char *ciphertext, const size_t ciphertext_len,
NULL,
ciphertext, (size_t) ciphertext_len,
tag,
NULL, 0,
aad, aad_len,
iv, key) != 0 ? 0 : 1;
}
@ -575,18 +612,40 @@ int encrypt_stream(const aead_func encrypt, const unsigned char *key, unsigned c
unsigned char *plaintext, unsigned char *ciphertext,
FILE *in, FILE *out, FILE *err
) {
size_t plaintext_read;
unsigned char aad[1] = {0};
unsigned char* plaintext_read_zone = plaintext + 1;
size_t plaintext_read, extra_byte_idx = buffer_size - 1;
while ((plaintext_read = fread(plaintext, 1, buffer_size, in)) > 0) {
/* read single byte */
plaintext_read = fread(plaintext, 1, 1, in);
if(plaintext_read != 1) {
if(NULL != err)
fprintf(err, "Cowardly refusing to encrypt empty file...\n");
return 0;
}
if(1 != encrypt(plaintext, plaintext_read, key, iv, ciphertext, ciphertext + plaintext_read))
while ((plaintext_read = fread(plaintext_read_zone, 1, buffer_size, in)) == buffer_size) {
/* there are more chunks after this */
if(1 != encrypt(plaintext, plaintext_read, key, iv, NULL, 0, ciphertext, ciphertext + plaintext_read))
return 0;
/* save extra byte at end */
plaintext[0] = plaintext_read_zone[extra_byte_idx];
if(1 != iv_increment_forbid_zero(iv, IV_LEN, err))
return 0;
fwrite(ciphertext, 1, plaintext_read + AEAD_TAG_LEN, out);
}
/* this is the last chunk, use AD */
plaintext_read += 1;
if(1 != encrypt(plaintext, plaintext_read, key, iv, aad, 1, ciphertext, ciphertext + plaintext_read))
return 0;
fwrite(ciphertext, 1, plaintext_read + AEAD_TAG_LEN, out);
return 1;
}
@ -595,11 +654,20 @@ int decrypt_stream(const aead_func decrypt, const unsigned char *key, unsigned c
unsigned char *plaintext, unsigned char *ciphertext,
FILE *in, FILE *out, FILE *err
) {
size_t ciphertext_read;
unsigned char aad[1] = {0};
unsigned char* ciphertext_read_zone = ciphertext + 1;
size_t ciphertext_read, extra_byte_idx = buffer_size + AEAD_TAG_LEN - 1;
buffer_size += AEAD_TAG_LEN;
while ((ciphertext_read = fread(ciphertext, 1, buffer_size, in)) > 0) {
ciphertext_read = fread(ciphertext, 1, 1, in);
if(ciphertext_read != 1) {
if(NULL != err)
fprintf(err, "File too small for decryption, truncated?\n");
return 0;
}
while ((ciphertext_read = fread(ciphertext_read_zone, 1, buffer_size, in)) == buffer_size) {
if(ciphertext_read < AEAD_TAG_LEN) {
if(NULL != err)
@ -609,14 +677,33 @@ int decrypt_stream(const aead_func decrypt, const unsigned char *key, unsigned c
ciphertext_read -= AEAD_TAG_LEN;
if(1 != decrypt(ciphertext, ciphertext_read, key, iv, ciphertext + ciphertext_read, plaintext))
return 0;
if(1 != decrypt(ciphertext, ciphertext_read, key, iv, NULL, 0, ciphertext + ciphertext_read, plaintext))
return 0;
/* save extra byte at end */
ciphertext[0] = ciphertext_read_zone[extra_byte_idx];
if(1 != iv_increment_forbid_zero(iv, IV_LEN, err))
return 0;
fwrite(plaintext, 1, ciphertext_read, out);
}
/* this is the last chunk, use AD */
ciphertext_read += 1;
if(ciphertext_read < AEAD_TAG_LEN) {
if(NULL != err)
fprintf(err, "File too small for decryption, truncated?\n");
return 0;
}
ciphertext_read -= AEAD_TAG_LEN;
if(1 != decrypt(ciphertext, ciphertext_read, key, iv, aad, 1, ciphertext + ciphertext_read, plaintext))
return 0;
fwrite(plaintext, 1, ciphertext_read, out);
return 1;
}
@ -665,13 +752,13 @@ int stream(const stream_func crypt_stream, const aead_func aead, unsigned char *
return 0;
}
plaintext = malloc(buffer_size);
plaintext = malloc(buffer_size + 1);
if(!plaintext) {
if(NULL != err)
fprintf(err, "plaintext memory allocation failed\n");
return 0;
}
ciphertext = malloc(buffer_size + AEAD_TAG_LEN);
ciphertext = malloc(buffer_size + AEAD_TAG_LEN + 1);
if(!ciphertext) {
if(NULL != err)
fprintf(err, "ciphertext memory allocation failed\n");