Crypto libraries
OpenSSL (cont.)
Milan Brož
xbroz@fi.muni.cz
PV181, FI MUNI, Brno
OpenSSL – www.openssl.org
 opensource cryptography toolkit
 Apache-style license
 hash, symmetric/asymmetric encryption, PKI, CA, ...
 ASN.1, PKCS-5,7,8,12, X509, OCSP, PEM
 SSL and TLS
 command line tool
 C/C++ library bindings (+many other library wrappers)
 on Linux compile with -lcrypto –lssl
 #include <openssl/...>
Today’s exercise
 Continue with OpenSSL on Linux
 Symmetric Encryption
 Demonstration of failures in some modes
 BIO (I/O abstraction)
 Assignment (see separate file, 10 points)
Example 4:
Symmetric encryption
OpenSSL
Encryption with EVP interface. Cipher mode is for example EVP_aes_256_cbc().
EVP_CIPHER_CTX_new()
EVP_EncryptInit_ex(context, EVP_cipher_mode,
NULL/*engine*/, key, iv)
EVP_EncryptUpdate(context, ciphertext, &clen, plaintext, plen)
EVP_EncryptFinal_ex(context, ciphertext + clen, &len)
EVP_CIPHER_CTX_free(context)
See 4_encryption_openssl directory.
Symmetric encryption:
ciphertext
ECB mode
…should be never used 
Wrong use demo: ciphertext patterns, block relocation.
See 4a_encryption_fails_openssl directory.
picture: Wikipedia
XTS mode
storage (file, disk) encryption
Wrong use demo: block patterns with constant IV.
See 4a_encryption_fails_openssl directory.
picture: Wikipedia
Symmetric encryption fails:
patterns in ciphertext
CBC mode
Wrong use demo: first block bit flips (IV) and consecutive block change.
See 4a_encryption_fails_openssl directory.
picture: Wikipedia
CTR mode
(counter mode)
Wrong use demo: re-use key from known ciphertext/plaintext pair.
See 4a_encryption_fails_openssl directory.
picture: Wikipedia
No integrity protection…
(no authentication of data)
Authenticated mode
GCM - Galois/Counter Mode
See 4a_encryption_fails_openssl directory.
picture: Wikipedia
Authenticated Encryption
with Additional Data (AEAD):
confidentiality + integrity.
- additional auth. data (AAD)
- data (plaintext/ciphertext)
- authentication tag
Example 5:
OpenSSL BIO (I/O abstraction)
Source/sink BIOs:
BIO_s_mem() - memory I/O
BIO_s_file() - file I/O
BIO_s_fd() - file descriptor IO
BIO_s_socket() - sockets
BIO_s_accept()
BIO_s_connect()
BIO_s_null() - discard (like /dev/null)
Filters
BIO_f_base64() - Base64 encoding
BIO_f_buffer() - buffering I/O
BIO_f_cipher() - encryption/decryption
BIO_f_md() - message digest
BIO_f_ssl() - SSL support for BIO
Example 5: the same encryption as in Example 4 using BIO interface.
See 5_bio_openssl directory.
Assignment
 Two goals:
 Use symmetric encryption (AES128-CBC) for a file.
 Self-study OpenSSL manual/wiki and examples.
 Max. 10 points.
 See Assignment.txt in IS for details and deadline
 You can start with examples in git
 Comment your code
 Note: for encryption expect binary input
 You can use provided Fedora VM or aisa server
(or any OpenSSL Linux, even Win10 embedded Linux)