11.13. Encryption and Compression Functions

Table 11.17. Encryption Functions

NameDescription
AES_DECRYPT()Decrypt using AES
AES_ENCRYPT()Encrypt using AES
COMPRESS()Return result as a binary string
DECODE()Decodes a string encrypted using ENCODE()
DES_DECRYPT()Decrypt a string
DES_ENCRYPT()Encrypt a string
ENCODE()Encode a string
ENCRYPT()Encrypt a string
MD5()Calculate MD5 checksum
OLD_PASSWORD()Return the value of the pre-4.1 implementation of PASSWORD
PASSWORD()Calculate and return a password string
SHA1(), SHA()Calculate an SHA-1 160-bit checksum
SHA2()Calculate an SHA-2 checksum
UNCOMPRESS()Uncompress a string compressed
UNCOMPRESSED_LENGTH()Return the length of a string before compression

Many encryption and compression functions return strings for which the result might contain arbitrary byte values. If you want to store these results, use a column with a VARBINARY or BLOB binary string data type. This will avoid potential problems with trailing space removal or character set conversion that would change data values, such as may occur if you use a nonbinary string data type (CHAR, VARCHAR, TEXT).

Some encryption functions return strings of ASCII characters: MD5(), OLD_PASSWORD(), PASSWORD(), SHA(), SHA1(). As of MySQL 5.5.3, their return value is a nonbinary string that has a character set and collation determined by the character_set_connection and collation_connection system variables. Before 5.5.3, these functions return binary strings. The same change was made for SHA2() in MySQL 5.5.6.

For versions in which functions such as MD5() or SHA1() return a string of hex digits as a binary string, the return value cannot be converted to uppercase or compared in case-insensitive fashion as is. You must convert the value to a nonbinary string. See the discussion of binary string conversion in Section 11.10, “Cast Functions and Operators”.

If an application stores values from a function such as MD5() or SHA1() that returns a string of hex digits, more efficient storage and comparisons can be obtained by converting the hex representation to binary using UNHEX() and storing the result in a BINARY(N) column. Each pair of hex digits requires one byte in binary form, so the value of N depends on the length of the hex string. N is 16 for an MD5() value and 20 for a SHA1() value. For SHA2(), N ranges from 28 to 32 depending on the argument specifying the desired bit length of the result.

The size penalty for storing the hex string in a CHAR column is at least two times, up to eight times if the value is stored in a column that uses the utf8 character set (where each character uses 4 bytes). Storing the string also results in slower comparisons because of the larger values and the need to take character set collation rules into account.

Suppose that an application stores MD5() string values in a CHAR(32) column:

CREATE TABLE md5_tbl (md5_val CHAR(32), ...);
INSERT INTO md5_tbl (md5_val, ...) VALUES(MD5('abcdef'), ...);

To convert hex strings to more compact form, modify the application to use UNHEX() and BINARY(16) instead as follows:

CREATE TABLE md5_tbl (md5_val BINARY(16), ...);
INSERT INTO md5_tbl (md5_val, ...) VALUES(UNHEX(MD5('abcdef')), ...);

Applications should be prepared to handle the very rare case that a hashing function produces the same value for two different input values. One way to make collisions detectable is to make the hash column a primary key.

Note

Exploits for the MD5 and SHA-1 algorithms have become known. You may wish to consider using one of the other encryption functions described in this section instead, such as SHA2().

Caution

Passwords or other sensitive values supplied as arguments to encryption functions are sent in plaintext to the MySQL server unless an SSL connection is used. Also, such values will appear in any MySQL logs to which they are written. To avoid these types of exposure, applications can encrypt sensitive values on the client side before sending them to the server. The same considerations apply to encryption keys. To avoid exposing these, applications can use stored procedures to encrypt and decrypt values on the server side.

  • AES_DECRYPT(crypt_str,key_str)

    This function decrypts data using the official AES (Advanced Encryption Standard) algorithm. For more information, see the description of AES_ENCRYPT().

  • AES_ENCRYPT(str,key_str)

    AES_ENCRYPT() and AES_DECRYPT() enable encryption and decryption of data using the official AES (Advanced Encryption Standard) algorithm, previously known as “Rijndael.” Encoding with a 128-bit key length is used, but you can extend it up to 256 bits by modifying the source. We chose 128 bits because it is much faster and it is secure enough for most purposes.

    AES_ENCRYPT() encrypts a string and returns a binary string. AES_DECRYPT() decrypts the encrypted string and returns the original string. The input arguments may be any length. If either argument is NULL, the result of this function is also NULL.

    Because AES is a block-level algorithm, padding is used to encode uneven length strings and so the result string length may be calculated using this formula:

    16 * (trunc(string_length / 16) + 1)
    

    If AES_DECRYPT() detects invalid data or incorrect padding, it returns NULL. However, it is possible for AES_DECRYPT() to return a non-NULL value (possibly garbage) if the input data or the key is invalid.

    You can use the AES functions to store data in an encrypted form by modifying your queries:

    INSERT INTO t VALUES (1,AES_ENCRYPT('text','password'));
    

    AES_ENCRYPT() and AES_DECRYPT() can be considered the most cryptographically secure encryption functions currently available in MySQL.

  • COMPRESS(string_to_compress)

    Compresses a string and returns the result as a binary string. This function requires MySQL to have been compiled with a compression library such as zlib. Otherwise, the return value is always NULL. The compressed string can be uncompressed with UNCOMPRESS().

    mysql> SELECT LENGTH(COMPRESS(REPEAT('a',1000)));
            -> 21
    mysql> SELECT LENGTH(COMPRESS(''));
            -> 0
    mysql> SELECT LENGTH(COMPRESS('a'));
            -> 13
    mysql> SELECT LENGTH(COMPRESS(REPEAT('a',16)));
            -> 15
    

    The compressed string contents are stored the following way:

    • Empty strings are stored as empty strings.

    • Nonempty strings are stored as a four-byte length of the uncompressed string (low byte first), followed by the compressed string. If the string ends with space, an extra “.” character is added to avoid problems with endspace trimming should the result be stored in a CHAR or VARCHAR column. (However, use of nonbinary string data types such as CHAR or VARCHAR to store compressed strings is not recommended anyway because character set conversion may occur. Use a VARBINARY or BLOB binary string column instead.)

  • DECODE(crypt_str,pass_str)

    Decrypts the encrypted string crypt_str using pass_str as the password. crypt_str should be a string returned from ENCODE().

  • DES_DECRYPT(crypt_str[,key_str])

    Decrypts a string encrypted with DES_ENCRYPT(). If an error occurs, this function returns NULL.

    This function works only if MySQL has been configured with SSL support. See Section 5.5.8, “Using SSL for Secure Connections”.

    If no key_str argument is given, DES_DECRYPT() examines the first byte of the encrypted string to determine the DES key number that was used to encrypt the original string, and then reads the key from the DES key file to decrypt the message. For this to work, the user must have the SUPER privilege. The key file can be specified with the --des-key-file server option.

    If you pass this function a key_str argument, that string is used as the key for decrypting the message.

    If the crypt_str argument does not appear to be an encrypted string, MySQL returns the given crypt_str.

  • DES_ENCRYPT(str[,{key_num|key_str}])

    Encrypts the string with the given key using the Triple-DES algorithm.

    This function works only if MySQL has been configured with SSL support. See Section 5.5.8, “Using SSL for Secure Connections”.

    The encryption key to use is chosen based on the second argument to DES_ENCRYPT(), if one was given. With no argument, the first key from the DES key file is used. With a key_num argument, the given key number (0 to 9) from the DES key file is used. With a key_str argument, the given key string is used to encrypt str.

    The key file can be specified with the --des-key-file server option.

    The return string is a binary string where the first character is CHAR(128 | key_num). If an error occurs, DES_ENCRYPT() returns NULL.

    The 128 is added to make it easier to recognize an encrypted key. If you use a string key, key_num is 127.

    The string length for the result is given by this formula:

    new_len = orig_len + (8 - (orig_len % 8)) + 1
    

    Each line in the DES key file has the following format:

    key_num des_key_str
    

    Each key_num value must be a number in the range from 0 to 9. Lines in the file may be in any order. des_key_str is the string that is used to encrypt the message. There should be at least one space between the number and the key. The first key is the default key that is used if you do not specify any key argument to DES_ENCRYPT().

    You can tell MySQL to read new key values from the key file with the FLUSH DES_KEY_FILE statement. This requires the RELOAD privilege.

    One benefit of having a set of default keys is that it gives applications a way to check for the existence of encrypted column values, without giving the end user the right to decrypt those values.

    mysql> SELECT customer_address FROM customer_table 
         > WHERE crypted_credit_card = DES_ENCRYPT('credit_card_number');
    
  • ENCODE(str,pass_str)

    Encrypt str using pass_str as the password. To decrypt the result, use DECODE().

    The result is a binary string of the same length as str.

    The strength of the encryption is based on how good the random generator is. It should suffice for short strings.

  • ENCRYPT(str[,salt])

    Encrypts str using the Unix crypt() system call and returns a binary string. The salt argument must be a string with at least two characters or the result will be NULL. If no salt argument is given, a random value is used.

    mysql> SELECT ENCRYPT('hello');
            -> 'VxuFAJXVARROc'
    

    ENCRYPT() ignores all but the first eight characters of str, at least on some systems. This behavior is determined by the implementation of the underlying crypt() system call.

    The use of ENCRYPT() with the ucs2, utf16, or utf32 multi-byte character sets is not recommended because the system call expects a string terminated by a zero byte.

    If crypt() is not available on your system (as is the case with Windows), ENCRYPT() always returns NULL.

  • MD5(str)

    Calculates an MD5 128-bit checksum for the string. The value is returned as a string of 32 hex digits, or NULL if the argument was NULL. The return value can, for example, be used as a hash key. See the notes at the beginning of this section about storing hash values efficiently.

    As of MySQL 5.5.3, the return value is a nonbinary string in the connection character set. Before 5.5.3, the return value is a binary string; see the notes at the beginning of this section about using the value as a nonbinary string.

    mysql> SELECT MD5('testing');
            -> 'ae2b1fca515949e5d54fb22b8ed95575'
    

    This is the “RSA Data Security, Inc. MD5 Message-Digest Algorithm.

    See the note regarding the MD5 algorithm at the beginning this section.

  • OLD_PASSWORD(str)

    OLD_PASSWORD() was added when the implementation of PASSWORD() was changed in MySQL 4.1 to improve security. OLD_PASSWORD() returns the value of the pre-4.1 implementation of PASSWORD() as a string, and is intended to permit you to reset passwords for any pre-4.1 clients that need to connect to your version 5.5 MySQL server without locking them out. See Section 5.3.2.3, “Password Hashing in MySQL”.

    As of MySQL 5.5.3, the return value is a nonbinary string in the connection character set. Before 5.5.3, the return value is a binary string.

  • PASSWORD(str)

    Calculates and returns a password string from the plaintext password str and returns a string, or NULL if the argument was NULL. This is the function that is used for encrypting MySQL passwords for storage in the Password column of the user grant table.

    As of MySQL 5.5.3, the return value is a nonbinary string in the connection character set. Before 5.5.3, the return value is a binary string.

    mysql> SELECT PASSWORD('badpwd');
            -> '*AAB3E285149C0135D51A520E1940DD3263DC008C'
    

    PASSWORD() encryption is one-way (not reversible).

    PASSWORD() does not perform password encryption in the same way that Unix passwords are encrypted. See ENCRYPT().

    Note

    The PASSWORD() function is used by the authentication system in MySQL Server; you should not use it in your own applications. For that purpose, consider MD5() or SHA2() instead. Also see RFC 2195, section 2 (Challenge-Response Authentication Mechanism (CRAM)), for more information about handling passwords and authentication securely in your applications.

    Important

    Statements that invoke PASSWORD() may be recorded in server logs or in a history file such as ~/.mysql_history, which means that plaintext passwords may be read by anyone having read access to that information. See Section 5.3.2, “Password Security in MySQL”.

  • SHA1(str), SHA(str)

    Calculates an SHA-1 160-bit checksum for the string, as described in RFC 3174 (Secure Hash Algorithm). The value is returned as a string of 40 hex digits, or NULL if the argument was NULL. One of the possible uses for this function is as a hash key. See the notes at the beginning of this section about storing hash values efficiently. You can also use SHA1() as a cryptographic function for storing passwords. SHA() is synonymous with SHA1().

    As of MySQL 5.5.3, the return value is a nonbinary string in the connection character set. Before 5.5.3, the return value is a binary string; see the notes at the beginning of this section about using the value as a nonbinary string.

    mysql> SELECT SHA1('abc');
            -> 'a9993e364706816aba3e25717850c26c9cd0d89d'
    

    SHA1() can be considered a cryptographically more secure equivalent of MD5(). However, see the note regarding the MD5 and SHA-1 algorithms at the beginning this section.

  • SHA2(str, hash_length)

    Calculates the SHA-2 family of hash functions (SHA-224, SHA-256, SHA-384, and SHA-512). The first argument is the cleartext string to be hashed. The second argument indicates the desired bit length of the result, which must have a value of 224, 256, 384, 512, or 0 (which is equivalent to 256). If either argument is NULL or the hash length is not one of the permitted values, the return value is NULL. Otherwise, the function result is a hash value containing the desired number of bits. See the notes at the beginning of this section about storing hash values efficiently.

    As of MySQL 5.5.6, the return value is a nonbinary string in the connection character set. Before 5.5.6, the return value is a binary string; see the notes at the beginning of this section about using the value as a nonbinary string.

    mysql> SELECT SHA2('abc', 224);
            -> '23097d223405d8228642a477bda255b32aadbce4bda0b3f7e36c9da7'
    

    This function works only if MySQL has been configured with SSL support. See Section 5.5.8, “Using SSL for Secure Connections”.

    SHA2() can be considered cryptographically more secure than MD5() or SHA1().

    SHA2() was added in MySQL 5.5.5.

  • UNCOMPRESS(string_to_uncompress)

    Uncompresses a string compressed by the COMPRESS() function. If the argument is not a compressed value, the result is NULL. This function requires MySQL to have been compiled with a compression library such as zlib. Otherwise, the return value is always NULL.

    mysql> SELECT UNCOMPRESS(COMPRESS('any string'));
            -> 'any string'
    mysql> SELECT UNCOMPRESS('any string');
            -> NULL
    
  • UNCOMPRESSED_LENGTH(compressed_string)

    Returns the length that the compressed string had before being compressed.

    mysql> SELECT UNCOMPRESSED_LENGTH(COMPRESS(REPEAT('a',30)));
            -> 30
    
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