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12.13 Encryption and Compression Functions

Table 12.17 Encryption Functions

Name Description
AES_DECRYPT() Decrypt using AES
AES_ENCRYPT() Encrypt using AES
ASYMMETRIC_DECRYPT() Decrypt ciphertext using private or public key
ASYMMETRIC_DERIVE() Derive symmetric key from asymmetric keys
ASYMMETRIC_ENCRYPT() Encrypt cleartext using private or public key
ASYMMETRIC_SIGN() Generate signature from digest
ASYMMETRIC_VERIFY() Verify that signature matches digest
COMPRESS() Return result as a binary string
CREATE_DH_PARAMETERS() Generate shared DH secret
CREATE_DIGEST() Generate digest from string
DECODE() Decode 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
PASSWORD() Calculate and return a password string
RANDOM_BYTES() Return a random byte vector
SHA1(), SHA() Calculate an SHA-1 160-bit checksum
SHA2() Calculate an SHA-2 checksum
STATEMENT_DIGEST() Compute statement digest hash value
STATEMENT_DIGEST_TEXT() Compute normalized statement digest
UNCOMPRESS() Uncompress a string compressed
UNCOMPRESSED_LENGTH() Return the length of a string before compression
VALIDATE_PASSWORD_STRENGTH() Determine strength of password

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(), SHA(), SHA1(), SHA2(), STATEMENT_DIGEST(), STATEMENT_DIGEST_TEXT(). Their return value is a string that has a character set and collation determined by the character_set_connection and collation_connection system variables. This is a nonbinary string unless the character set is binary.

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 hexadecimal 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:

  1. CREATE TABLE md5_tbl (md5_val CHAR(32), ...);
  2. 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:

  1. CREATE TABLE md5_tbl (md5_val BINARY(16), ...);
  2. 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.


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


Passwords or other sensitive values supplied as arguments to encryption functions are sent as cleartext 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[,init_vector])

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

    The optional initialization vector argument, init_vector. Statements that use AES_DECRYPT() are unsafe for statement-based replication.

  • AES_ENCRYPT(str,key_str[,init_vector])

    AES_ENCRYPT() and AES_DECRYPT() implement encryption and decryption of data using the official AES (Advanced Encryption Standard) algorithm, previously known as Rijndael. The AES standard permits various key lengths. By default these functions implement AES with a 128-bit key length. Key lengths of 196 or 256 bits can be used, as described later. The key length is a trade off between performance and security.

    AES_ENCRYPT() encrypts the string str using the key string key_str and returns a binary string containing the encrypted output. AES_DECRYPT() decrypts the encrypted string crypt_str using the key string key_str and returns the original plaintext string. If either function argument is NULL, the function returns NULL.

    The str and crypt_str arguments can be any length, and padding is automatically added to str so it is a multiple of a block as required by block-based algorithms such as AES. This padding is automatically removed by the AES_DECRYPT() function. The length of crypt_str can be calculated using this formula:

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

    For a key length of 128 bits, the most secure way to pass a key to the key_str argument is to create a truly random 128-bit value and pass it as a binary value. For example:

    1. VALUES (1,AES_ENCRYPT('text',UNHEX('F3229A0B371ED2D9441B830D21A390C3')));

    A passphrase can be used to generate an AES key by hashing the passphrase. For example:

    1. VALUES (1,AES_ENCRYPT('text', UNHEX(SHA2('My secret passphrase',512))));

    Do not pass a password or passphrase directly to crypt_str, hash it first. Previous versions of this documentation suggested the former approach, but it is no longer recommended as the examples shown here are more secure.

    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.

    AES_ENCRYPT() and AES_DECRYPT() permit control of the block encryption mode and take an optional init_vector initialization vector argument:

    • The block_encryption_mode system variable controls the mode for block-based encryption algorithms. Its default value is aes-128-ecb, which signifies encryption using a key length of 128 bits and ECB mode. For a description of the permitted values of this variable, see Section 5.1.8, “Server System Variables”.

    • The optional init_vector argument provides an initialization vector for block encryption modes that require it.

    For modes that require the optional init_vector argument, it must be 16 bytes or longer (bytes in excess of 16 are ignored). An error occurs if init_vector is missing.

    For modes that do not require init_vector, it is ignored and a warning is generated if it is specified.

    A random string of bytes to use for the initialization vector can be produced by calling RANDOM_BYTES(16). For encryption modes that require an initialization vector, the same vector must be used for encryption and decryption.

    1. mysql> SET block_encryption_mode = 'aes-256-cbc';
    2. mysql> SET @key_str = SHA2('My secret passphrase',512);
    3. mysql> SET @init_vector = RANDOM_BYTES(16);
    4. mysql> SET @crypt_str = AES_ENCRYPT('text',@key_str,@init_vector);
    5. mysql> SELECT AES_DECRYPT(@crypt_str,@key_str,@init_vector);
    6. +-----------------------------------------------+
    7. | AES_DECRYPT(@crypt_str,@key_str,@init_vector) |
    8. +-----------------------------------------------+
    9. | text                                          |
    10. +-----------------------------------------------+

    The following table lists each permitted block encryption mode, the SSL libraries that support it, and whether the initialization vector argument is required.

    Block Encryption Mode SSL Libraries that Support Mode Initialization Vector Required
    ECB OpenSSL, wolfSSL No
    CBC OpenSSL, wolfSSL Yes
    CFB1 OpenSSL Yes
    CFB8 OpenSSL Yes
    CFB128 OpenSSL Yes
    OFB OpenSSL Yes

    Statements that use AES_ENCRYPT() or AES_DECRYPT() are unsafe for statement-based replication.

  • 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().

    1. mysql> SELECT LENGTH(COMPRESS(REPEAT('a',1000)));
    2.         -> 21
    3. mysql> SELECT LENGTH(COMPRESS(''));
    4.         -> 0
    5. mysql> SELECT LENGTH(COMPRESS('a'));
    6.         -> 13
    7. mysql> SELECT LENGTH(COMPRESS(REPEAT('a',16)));
    8.         -> 15

    The compressed string contents are stored the following way:

    • Empty strings are stored as empty strings.

    • Nonempty strings are stored as a 4-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)

    This function was removed in MySQL 8.0.3.

    Consider using AES_ENCRYPT() and AES_DECRYPT() instead.

  • DES_DECRYPT(crypt_str[,key_str])

    This function was removed in MySQL 8.0.3.

    Consider using AES_ENCRYPT() and AES_DECRYPT() instead.

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

    This function was removed in MySQL 8.0.3.

    Consider using AES_ENCRYPT() and AES_DECRYPT() instead.

  • ENCODE(str,pass_str)

    This function was removed in MySQL 8.0.3.

    Consider using AES_ENCRYPT() and AES_DECRYPT() instead.

  • ENCRYPT(str[,salt])

    This function was removed in MySQL 8.0.3. For one-way hashing, consider using SHA2() instead.

  • MD5(str)

    Calculates an MD5 128-bit checksum for the string. The value is returned as a string of 32 hexadecimal 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.

    The return value is a string in the connection character set.

    If FIPS mode is enabled, MD5() returns NULL. See Section 6.5, “FIPS Support”.

    1. mysql> SELECT MD5('testing');
    2.         -> 'ae2b1fca515949e5d54fb22b8ed95575'

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

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

  • PASSWORD(str)

    This function was removed in MySQL 8.0.11.


    This function returns a binary string of len random bytes generated using the random number generator of the SSL library. Permitted values of len range from 1 to 1024. For values outside that range, RANDOM_BYTES() generates a warning and returns NULL.

    RANDOM_BYTES() can be used to provide the initialization vector for the AES_DECRYPT() and AES_ENCRYPT() functions. For use in that context, len must be at least 16. Larger values are permitted, but bytes in excess of 16 are ignored.

    RANDOM_BYTES() generates a random value, which makes its result nondeterministic. Consequently, statements that use this function are unsafe for statement-based replication.

  • 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 hexadecimal 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. SHA() is synonymous with SHA1().

    The return value is a string in the connection character set.

    1. mysql> SELECT SHA1('abc');
    2.         -> '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 plaintext 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.

    The return value is a string in the connection character set.

    1. mysql> SELECT SHA2('abc', 224);
    2.         -> '23097d223405d8228642a477bda255b32aadbce4bda0b3f7e36c9da7'

    This function works only if MySQL has been configured with SSL support. See Section 6.3, “Using Encrypted Connections”.

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

  • STATEMENT_DIGEST(statement)

    Given an SQL statement as a string, returns the statement digest hash value as a string in the connection character set, or NULL if the argument is NULL. The related STATEMENT_DIGEST_TEXT() function returns the normalized statement digest. For information about statement digesting, see Section 26.10, “Performance Schema Statement Digests and Sampling”.

    Both functions use the MySQL parser to parse the statement. If parsing fails, an error occurs. The error message includes the parse error only if the statement is provided as a literal string.

    The max_digest_length system variable determines the maximum number of bytes available to these functions for computing normalized statement digests.

    1. mysql> SET @stmt = 'SELECT * FROM mytable WHERE cola = 10 AND colb = 20';
    2. mysql> SELECT STATEMENT_DIGEST(@stmt);
    3. +------------------------------------------------------------------+
    4. | STATEMENT_DIGEST(@stmt)                                          |
    5. +------------------------------------------------------------------+
    6. | 3bb95eeade896657c4526e74ff2a2862039d0a0fe8a9e7155b5fe492cbd78387 |
    7. +------------------------------------------------------------------+
    8. mysql> SELECT STATEMENT_DIGEST_TEXT(@stmt);
    9. +----------------------------------------------------------+
    10. | STATEMENT_DIGEST_TEXT(@stmt)                             |
    11. +----------------------------------------------------------+
    12. | SELECT * FROM `mytable` WHERE `cola` = ? AND `colb` = ?  |
    13. +----------------------------------------------------------+

    Given an SQL statement as a string, returns the normalized statement digest as a string in the connection character set, or NULL if the argument is NULL. For additional discussion and examples, see the description of the related STATEMENT_DIGEST() function.

  • 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.

    1. mysql> SELECT UNCOMPRESS(COMPRESS('any string'));
    2.         -> 'any string'
    3. mysql> SELECT UNCOMPRESS('any string');
    4.         -> NULL
  • UNCOMPRESSED_LENGTH(compressed_string)

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

    1.         -> 30

    Given an argument representing a plaintext password, this function returns an integer to indicate how strong the password is. The return value ranges from 0 (weak) to 100 (strong).

    Password assessment by VALIDATE_PASSWORD_STRENGTH() is done by the validate_password component. If that component is not installed, the function always returns 0. For information about installing validate_password, see Section 6.4.3, “The Password Validation Component”. To examine or configure the parameters that affect password testing, check or set the system variables implemented by validate_password. See Section, “Password Validation Options and Variables”.

    The password is subjected to increasingly strict tests and the return value reflects which tests were satisfied, as shown in the following table. In addition, if the validate_password.check_user_name system variable is enabled and the password matches the user name, VALIDATE_PASSWORD_STRENGTH() returns 0 regardless of how other validate_password system variables are set.

    Password Test Return Value
    Length < 4 0
    Length ≥ 4 and < validate_password.length 25
    Satisfies policy 1 (LOW) 50
    Satisfies policy 2 (MEDIUM) 75
    Satisfies policy 3 (STRONG) 100

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