OFB是一種加密模式。CFB和OFC工作模式中，密碼演算法象流密碼一樣生產出金鑰流供異或名密文用。

參考文獻

1. NIST SP 800-38A

1. OFB加解密

OFB 加密

I1 = IV;

Ij = Oj -1       for j = 2 … n;

Oj = CIPHK(Ij)    for j = 1, 2 … n;

Cj = Pj ⊕ Oj       for j = 1, 2 … n-1;

C*n= P*n ⊕ MSBu(On).

OFB 解密

I1 = IV;

Ij = Oj -1       for j = 2 … n;

Oj = CIPHK(Ij)    for j = 1, 2 … n;

Pj = Cj ⊕ Oj       for j = 1, 2 … n-1;

P*n = C*n ⊕ MSBu(On).

OFB模式

1. LibTomCrypt與OFB

LibTomCrypt中與OFB相關的資訊如下：

typedef struct {

int cipher;

int blocklen;

int padlen;

unsigned char IV[MAXBLOCKSIZE];

symmetric_key key;

} symmetric_OFB;

int ofb_start(int cipher, const unsigned char *IV, const unsigned char *key, int keylen, int num_rounds, symmetric_OFB *ofb);

int ofb_encrypt(const unsigned char *pt, unsigned char *ct, unsigned long len, symmetric_OFB *ofb);

int ofb_decrypt(const unsigned char *ct, unsigned char *pt, unsigned long len, symmetric_OFB *ofb);

int ofb_getiv(unsigned char *IV, unsigned long *len, symmetric_OFB *ofb);

int ofb_setiv(const unsigned char *IV, unsigned long len, symmetric_OFB *ofb);

int ofb_done(symmetric_OFB *ofb);

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int ofb_start(int cipher, const unsigned char *IV, const unsigned char *key, int keylen, int num_rounds, symmetric_OFB *ofb);

// [功能]   初始化

1. IV           // [輸入] 初始化向量
2. key              // [輸入] 金鑰
3. keylen     // [輸入] 金鑰長度
4. num_rounds  // [輸入] 輪數（建議設定為0以便讓系統使用預設值）
5. ofb          // [輸入/輸出] OFB狀態

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int ofb_encrypt(const unsigned char *pt, unsigned char *ct, unsigned long len, symmetric_OFB *ofb);

// [功能]   CBC加密

1. pt           // [輸入] 明文
2. ct            // [輸出] 密文
3. len          // [輸入] 明文密文長度
4. ofb          // [輸入/輸出] OFB狀態

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int ofb_decrypt(const unsigned char *ct, unsigned char *pt, unsigned long len, symmetric_OFB *ofb);

// [功能]   CBC解密

1. ct            // [輸入] 密文
2. pt           // [輸出] 明文
3. len          // [輸入] 明文密文長度
4. ofb          // [輸入/輸出] OFB狀態

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int ofb_getiv(unsigned char *IV, unsigned long *len, symmetric_OFB *ofb);

// [功能]   獲取IV值

1. IV           // [輸出] IV值
2. len          // [輸出] IV值長度
3. ofb          // [輸入/輸出] OFB狀態

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int ofb_setiv(const unsigned char *IV, unsigned long len, symmetric_OFB *ofb);

// [功能]   設定IV值

1. IV           // [輸入] IV值
2. len          // [輸入] IV值長度
3. ofb          // [輸入/輸出] OFB狀態

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int ofb_done(symmetric_OFB *ofb);

// [功能]   完成OFB

1. ofb          // [輸入/輸出] OFB狀態

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