diff options
Diffstat (limited to 'cbc.c')
| -rw-r--r-- | cbc.c | 81 |
1 files changed, 66 insertions, 15 deletions
@@ -49,6 +49,31 @@ cbc_encrypt(void *ctx, void (*f)(void *ctx, } } +/* Reqires that dst != src */ +static void +cbc_decrypt_internal(void *ctx, void (*f)(void *ctx, + unsigned length, uint8_t *dst, + const uint8_t *src), + unsigned block_size, uint8_t *iv, + unsigned length, uint8_t *dst, + const uint8_t *src) +{ + assert(length); + assert( !(length % block_size) ); + assert(src != dst); + + /* Decrypt in ECB mode */ + f(ctx, length, dst, src); + + /* XOR the cryptotext, shifted one block */ + memxor(dst, iv, block_size); + memxor(dst + block_size, src, length - block_size); + memcpy(iv, src + length - block_size, block_size); +} + +/* Don't allocate any more space than this on the stack */ +#define CBC_BUFFER_LIMIT 4096 + void cbc_decrypt(void *ctx, void (*f)(void *ctx, unsigned length, uint8_t *dst, @@ -62,23 +87,49 @@ cbc_decrypt(void *ctx, void (*f)(void *ctx, if (!length) return; - if (src == dst) + if (src != dst) + cbc_decrypt_internal(ctx, f, block_size, iv, + length, dst, src); + else { - /* Keep a copy of the ciphertext. */ - /* FIXME: If length is large enough, allocate a smaller buffer - * and process one buffer size at a time */ - uint8_t *tmp = alloca(length); - memcpy(tmp, src, length); - src = tmp; + /* We need a copy of the ciphertext, so we can't ECB decrypt in + * place. + * + * If length is small, we allocate a complete copy of src on the + * stack. Otherwise, we allocate a block of size at most + * CBC_BUFFER_LIMIT, and process that amount of data at a + * time. + * + * NOTE: We assume that block_size <= CBC_BUFFER_LIMIT. */ + + uint8_t *buffer; + + if (length <= CBC_BUFFER_LIMIT) + buffer = alloca(length); + else + { + /* The buffer size must be an integral number of blocks. */ + unsigned buffer_size + = CBC_BUFFER_LIMIT - (CBC_BUFFER_LIMIT % block_size); + + buffer = alloca(buffer_size); + + for ( ; length >= buffer_size; + length -= buffer_size, dst += buffer_size, src += buffer_size) + { + memcpy(buffer, src, buffer_size); + cbc_decrypt_internal(ctx, f, block_size, iv, + buffer_size, dst, buffer); + } + if (!length) + return; + } + /* Now, we have at most CBC_BUFFER_LIMIT octets left */ + memcpy(buffer, src, length); + + cbc_decrypt_internal(ctx, f, block_size, iv, + length, dst, buffer); } - - /* Decrypt in ECB mode */ - f(ctx, length, dst, src); - - /* XOR the cryptotext, shifted one block */ - memxor(dst, iv, block_size); - memxor(dst + block_size, src, length - block_size); - memcpy(iv, src + length - block_size, block_size); } #if 0 |