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In CTR mode, support to encrypt/decrypt from the middle of a message #1077

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In CTR mode, support to encrypt/decrypt from the middle of a message #1077

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7 changes: 5 additions & 2 deletions lib/cipherModes.js
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Original file line number Diff line number Diff line change
Expand Up @@ -446,6 +446,9 @@ modes.ctr.prototype.start = function(options) {
// use IV as first input
this._iv = transformIV(options.iv, this.blockSize);
this._inBlock = this._iv.slice(0);
if(options.counter) {
inc32(this._inBlock, options.counter);
}
this._partialBytes = 0;
};

Expand Down Expand Up @@ -988,9 +991,9 @@ function transformIV(iv, blockSize) {
return iv;
}

function inc32(block) {
function inc32(block, count) {
// increment last 32 bits of block only
block[block.length - 1] = (block[block.length - 1] + 1) & 0xFFFFFFFF;
block[block.length - 1] = (block[block.length - 1] + (count || 1)) & 0xFFFFFFFF;
}

function from64To32(num) {
Expand Down