user3667: How do sites like blockchain.info and bitaddress.org generate private keys by simply hashing a string? (the passphrase?) 
In this wiki article,
https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addresses
Step "0" starts with an ECDSA key pair already generated.
Does this mean that any 256-bit value can be a valid ECDSA private key? So wherever I am in the world, I can always just SHA256(passphrase) and get my private key? 

user10688: We made a much improved version of brainwallet that uses scrypt key-stretching to protect your secret key.  See it here: https://keybase.io/warp.  There's currently an unbroken 20BTC challenge for an 8-letter passphrase.

user85: Any 256-bit value can be a valid secp256k1 ECDSA private key. Strictly speaking, there's an upper limit that's slightly lower than 2^256, but you can just wrap them around.
So yes, you can just use SHA256(passphrase) to generate a private key.

3667: <p>How do sites like blockchain.info and bitaddress.org generate private keys by simply hashing a string? (the passphrase?) </p>

<p>In this wiki article,
<a href="https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addresses" rel="nofollow">https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addresses</a>
Step "0" starts with an ECDSA key pair already generated.</p>

<p>Does this mean that any 256-bit value can be a valid ECDSA private key? So wherever I am in the world, I can always just SHA256(passphrase) and get my private key? </p>
 10688: <p>We made a much improved version of brainwallet that uses scrypt key-stretching to protect your secret key.  See it here: <a href="https://keybase.io/warp" rel="nofollow">https://keybase.io/warp</a>.  There's currently an unbroken 20BTC challenge for an 8-letter passphrase.</p>
 85: <p>Any 256-bit value can be a valid secp256k1 ECDSA private key. Strictly speaking, there's an upper limit that's slightly lower than 2^256, but you can just wrap them around.</p>

<p>So yes, you can just use SHA256(passphrase) to generate a private key.</p>
