Data & Network Security By. Samiullah Khan. Week 11. Hash Algorithm/Message Digest MD2 MD4

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1 Week 11 Hash Algorithm/Message Digest MD2 MD4

2 Hash Algorithm A Hash(Message Digest) is a one-way function Function which takes an input message & Produce an output Its not practical to figure out what input corresponds to a given output. Computational Infeasible to find a message that has a given prespecified message digest. Its impossible to find two messages that have same digest All digest/hash algorithms take an arbitrary-length message & windup with fixed length quantity. The algorithm "chops and mixes" (i.e., substitutes or transposes) the data to create such fingerprints, called hash values. In general terms, the principal object of A hash function is data integrity. A change to any bit or bits in results, with high probability, in a change to the hash code

3 Data & Network Security By

4 Commonly used Algorithms MD2 MD4 MD5 SHA-1 etc.

5 MD 2 Message Digest 2 (MD2) Need for a cryptographically secure message digest function Ron Rivest developed MD2 (RFC 1319), MD4 (RFC 1320), MD5 (RFC 1321) Later: SHS (Secure Hash Standard) MD2 was proprietary and was never published, MD3 was superseeded by MD4 The Input to MD is a message with an arbitrary number of bytes The message is padded to be a multiple of 16 bytes A 16-byte quantity called checksum is appended The MD is computed in a final pass

6 MD2 Padding There must always be padding (even if the length of the original message is a multiple of 16 bytes) If the length of message is a multiple of 16 bytes then add 16 bytes of padding Else add the necessary number of bytes to make the message a multiple of 16 bytes Each padding byte contains the number of padding bytes:

7 MD2 Checksum Computation Data & Network Security By

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9 Cont d The checksum is a 16-byte quantity. It is almost like a message digest, but it is not cryptographically secure by itself. The checksum is appended to the message and then MD2 processes the concatenated quantity to obtain the actual message. The checksum starts out initialized to 0. The message is a multiple of 16 bytes, say k x 16 bytes. The checksum calculation processes the padded message a byte at a time, so the calculation requires k x 16 steps.

10 Cont d Each step looks at one byte of the message and updates one byte of the checksum. After byte 15 of the checksum is updated, the next step starts again on byte 0 of the checksum. Therefore, each byte of the checksum will have been updated k times by the times the checksum calculation terminates. Refer RFC s

11 MD2 π Substitution Table Data & Network Security By

12 MD4 MD4 was designed to be 32-bit-word-oriented so that it can be computed faster on 32-bit CPU s than a byte-oriented scheme like MD2. MD2 requires the message to be an integral number of bytes. MD4 can handle message with an arbitrary number of bits. Like MD2 it can be computed in a single pass over the data, though MD4 needs more intermediate state.

13 MD4 Message Padding The message to be fed into the message digest computation must be a multiple of 512 bits (sixteen 32-bit words). The original message is padded by adding a 1 bit, followed by enough 0 bits to leave the message 64 bits less than a multiple of 512 bits. Then a 64-bit quantity representing the number of bits in the unpadded message, mod 2 64, is appended to message. Padding for MD4, MD5, SHA-1

14 MD4, MD5, SHA-1 Data & Network Security By

15 Cont d The message digest to be computed is a 128-bit quantity (four 32-bit-words). The message is processed in 512-bit (sixteen 32-bit words) blocks. The message digest is initialized to a fixed value, and then each stage of the message digest computation takes the current value of the message digest and modifies it using the next block of the message. The final result is the message digest for the entire message. Each stage makes three passes over the message block.

16 Cont d Each pass has a slightly different method of mangling the message digest. At the end of the stage, each word of the mangled message is added to its pre-stage value to produce the post-stage value (which becomes the pre-stage value for the next stage). Therefore, the current value of the message digest must be saved at the beginning of the stage so that it can be added in at the end of the stage. Each stage starts with a 16-word message block and a 4- word message digest value. The message words are called mo, m1,m2,., m15.