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Excerpted from NIST announcement of Draft Federal Information Processing Standard (FIPS) 46-3, Data Encryption Standard (DES), and Request for Comments: http://jya.com/nist011599.txt

### Triple Data Encryption Algorithm

Let EK(I) and DK(I) represent the DEA encryption and decryption of I using DEA key K respectively. Each TDEA encryption/decryption operation (as specified in ANSI X9.52) is a compound operation of DEA encryption and decryption operations. The following operations are used:

1. TDEA encryption operation: the transformation of a 64-bit block I into a 64-bit block ) that is defined as follows:

> O = EK3(DK2(EK1(I)))

2. TDEA decryption operation: the transformation of a 64-bit block I into a 64-bit block O that is defined as follows:

> O = DK1(EK2(DK3(I)))

The standard species the following keying options for bundle (K1, K2, K3)

> 1. Keying Option 1: K1, K2 and K3 are independent keys;

> 2. Keying option 2: K1 and K2 and [are?] independent keys and K3 = K1;

> 3. Keying Option 3: K1 = K2 = K3.

A TDEA mode of operation is backward compatible with its single DEA counterpart if, with compatible keying options for TDEA operation,

> 1. An encrypted plaintext computed using a single DEA mode of operation can be decrypted correctly by a corresponding TDEA mode of operation; and

> 2. An encrypted plaintext computed using a TDEA mode of operation can be decrypted correctly by a corresponding single DEA mode of operation.

When using keying Option 3 (K1 = K2 = K3), TECB, TCBC, TCFB, amd TOFB modes are backward compatible with single DEA modes of operation ECB, CBC, CFB, OFB respectively.

The diagram in Appendix 2 illustrates TDEA encryption an TDEA decrytion.

(Note that the two appendices to FIPS 46-3 are not reproduced in this Federal Register notice. They are available in the complete draft of FIPS 46-3.)

[Note: NIST says by telephone message at (301) 975-5237 bad weather has closed NIST and that draft of FIPS 46-3 is not yet available.]