W3C home > Mailing lists > Public > public-webcrypto@w3.org > June 2012

Re: Strawman proposal for the low-level API

From: Håvard Molland <haavardm@opera.com>
Date: Thu, 21 Jun 2012 10:34:57 +0200
Message-ID: <4FE2DCB1.4050203@opera.com>
To: public-webcrypto@w3.org
How about also incorporating the blob interface from [7]  ? 
(http://www.w3.org/TR/FileAPI/#dfn-Blob). The blob interface represents 
"immutable raw data". Since it doesn't make much sense to edit encrypted 
data, this is a nice fit with encryption APIs.

Without going into details, we could let processData take blob as input 
and allowing the encrypted result to read out as blob. And/or we could 
let a CryptoBlob object inheriting the Blob interface.

This would make it easy to encrypt files chosen by the user and to 
simultaneously stream and encrypt those files out on websockets or other 
interfaces that might use the blob interface in the feature.

Use case:

1) The script lets a user choose a file, and wraps the file blob into a 
CryptoBlob object.
2) The script sets the CryptoBlob in "encryption"  mode, and sets the 
key and algorithms.
3) The script sends the CryptoBlob to the server or another browser 
using websockets or any other api accepting the blob interface.
4) The receiver script either store the blob, or  wraps the received 
encrypted blob into a CryptoBlob object in "decryption" mode and 
decrypts it.

As the websockets implementations only see the blob interface, the 
effect is that the blob can be encrypted and streamed out 
simultaneously. Note that this would not be some replacement for TLS, as 
the data will stay encrypted on the receivers side until the receiver 
knows the key and decides to decrypt.

Cheers,
Håvard




On 06/18/2012 07:53 PM, Ryan Sleevi wrote:
> Hi all,
>
>    While I'm still in the process of learning WebIDL [1] and the W3C 
> Manual of Style [2], I wanted to take a quick shot at drafting a 
> strawman low-level API for discussion. We've discussed quite a bit 
> about key management [3] and key discovery [4], not to mention early 
> discussions about algorithm discovery [5], but I think it might be 
> good to move to the point where we can talk a bit about how these 
> might be all put together.
>
> First, a bit of the IDL definition, to set the stage. This is also 
> using using ArrayBuffer from TypedArray [6], which I'm not sure if 
> it's altogether appropriate, but it's been incorporated by reference 
> into FileAPI [7], so it seems alright to use here.
>
> [interface]
> interface CryptoStream : EventTarget {
>   void processData(ArrayBuffer buffer);
>   void processData(DOMString data);
>   void complete();
>
>   readonly attribute (DOMString or ArrayBuffer)? result;
>
>   attribute [TreatNonCallableAsNull] Function? onerror;
>   attribute [TreatNonCallableAsNull] Function? onprogress;
>   attribute [TreatNonCallableAsNull] Function? oncomplete;
> };
>
> dictionary AlgorithmParams {
> };
>
> dictionary Algorithm {
>   DOMString name;
>   AlgorithmParams? params;
> };
>
> [NoInterfaceObject]
> interface Crypto {
>   CryptoStream encrypt(Algorithm algorithm, Key key);
>   CryptoStream decrypt(Algorithm algorithm, Key key);
>
>   // Also handles MACs
>   CryptoStream sign(Algorithm algorithm, Key key);
>   CryptoStream verify(Algorithm algorithm, Key key, ArrayBuffer 
> signature);
>
>   CryptoStream digest(Algorithm algorithm);
>
>   // This interface TBD. See discussion below.
>   bool supports(Algorithm algorithm, optional Key key);
>
>   // Interfaces for key derivation/generation TBD.
> };
>
>
> As you can see, CryptoStream is used for all of the actual crypto 
> operations. That's because, in looking at the operations, I think all 
> of them will work on a series of calls to provide input, and the 
> result of which is either: error, some data output, or operation complete.
>
> The real challenge, I think, lies in the AlgorithmParams structure, 
> which is where all of the algorithm-specific magic happens. My belief 
> is that we can/should be able to define this API independent of any 
> specific AlgorithmParams - that is, we can define the generic state 
> machine, error handling, discovery. Then, as a supplemental work 
> (still within the scope of the primary goal), we define and enumerate 
> how exactly specific algorithms are implemented within this state machine.
>
> To show how different AlgorithmParams might be implemented, here's 
> some varies definitions:
>
> // For the 'RSA-PSS' algorithm.
> dictionary RsaPssParams : AlgorithmParams {
>   // The hashing function to apply to the message (eg: SHA1).
>   AlgorithmParams hash;
>   // The mask generation function (eg: MGF1-SHA1)
>    AlgorithmParams mgf;
>   // The desired length of the random salt.
>   unsigned long saltLength;
> };
>
> // For the 'RSA-OAEP' algorithm.
> dictionary RsaOaepParams : AlgorithmParams {
>   // The hash function to apply to the message (eg: SHA1).
>    AlgorithmParams hash;
>   // The mask generation function (eg: MGF1-SHA1).
>    AlgorithmParams mgf;
>   // The optional label/application data to associate with the signature.
>   DOMString? label = null;
> };
>
> // For the 'AES-GCM' algorithm.
> dictionary AesGcmParams : AlgorithmParams {
>   ArrayBufferView? iv;
>   ArrayBufferView? additional;
>   unsigned long tagLength;
> };
>
> // For the 'AES-CCM' algorithm.
> dictionary AesCcmParams : AlgorithmParams {
>   ArrayBufferView? nonce;
>   ArrayBufferView? additional;
>   unsigned long macLength;
> };
>
> // For the 'HMAC' algorithm.
> dictionary HmacParams : AlgorithmParams {
>   // The hash function to use (eg: SHA1).
>   AlgorithmParams hash;
> };
>
>
> The API behaviour is this:
> - If encrypt/decrypt/sign/verify/digest is called with an unsupported 
> algorithm, throw InvalidAlgorithmError.
> - If " is called with an invalid key, throw InvalidKeyError.
> - If " is called with an invalid key/algorithm combination, throw 
> UnsupportedAlgorithmError.
> - Otherwise, return a CryptoStream.
>
> For encrypt/decrypt
> - The caller calls processData() as data is available.
> - If the data can be en/decrypted, it will raise an onprogress event 
> (event type TBD).
>   - If new (plaintext, ciphertext) data is available, .result will be 
> updated. [This is similar to the FileStream API behaviour]
> - If the data cannot be en/decrypted, raise the onerror with an 
> appropriate error
> - The caller calls .complete() once all data has been processed.
>   - If the final block validates (eg: no padding errors), call 
> onprocess then oncomplete.
>   - If the final block does not validate, call onerror with an 
> appropriate error.
>
> For authenticated encryption modes, for example, the .result may not 
> contain any data until .complete has been called (with the result data).
>
> For sign/verify, it behaves similarly.
> - The caller calls processData() as data is available.
> - [No onprogress is called/needs to be called?]
> - The caller calls .complete() once all data has been processed
> - For sign, once .complete() is called, the signature is generated, 
> and either onprogress+oncomplete or onerror is called. If successful, 
> the resultant signature is in .result.
> - For verify, once .complete() is called, the signature is compared, 
> and either onprogress+oncomplete or onerror is called. If the 
> signatures successfully matched, .result will contain the input 
> signature (eg: the constant-time comparison happens within the 
> library). If the signatures don't match, .result will be null and the 
> error handler will have been called.
>
> Finally, for digesting, it behaves like .sign/.verify in that no data 
> is available until .complete() is called, and once .compete() is 
> called, the resultant digest is in .result.
>
> What I haven't fully worked out is how key derivation/agreement will 
> work - particularly if the result of some result of key agreement 
> results in multiple keys (eg: how SSL/TLS key derivation works in 
> PKCS#11). This is somewhat dependent on how we treat keys.
>
> Note that I left the Key type unspecified. It's not clear if this will 
> be something like (Key or DOMString), indicating some either/or of 
> handle / id, if it might be a dictionary type (with different naming 
> specifiers, such as 'id' or 'uuid'), or if it will be a concrete type 
> obtained via some other call (eg: .queryKeys()). I think that will be 
> borne out over the next week or two as we continue to discuss key 
> management/lifecycle.
>
> For a pseudo-code example:
>
> var stream = window.crypto.sign({ name: 'RSA-PSS', params: { hash: { 
> name: 'sha1' }, mgf: { name: 'mgf-sha1' }, saltLength: 32 }}, key);
> stream.oncomplete = function(evt) { window.alert('The signature is ' + 
> e.target.result); };
> stream.onerror = function(evt) { window.alert('Signing caused an 
> error: ' + e.error); };
>
> var filereader = FileReader();
> reader.onload = function(evt) { stream.processData(evt.target.result); 
> stream.complete(); }
> filereader.readAsArrayBuffer(someFile);
>
>
> The FileAPI is probably not the best example of why the iterative API 
> (.processData() + .complete()) is used, since FileReader has the 
> FileReader.result containing all of the processed data, but it's 
> similar than demonstrating a streaming operation that may be using 
> WebSockets [8] or PeerConnection [9].
>
> Note that I think during the process of algorithm specification, we 
> can probably get away with also defining well-known shorthand. eg: 
> 'RSA-PSS-SHA256' would mean that the hash is SHA-256, the mgf is 
> MGF1-SHA256, and only the saltLength needs to be specified (or should 
> it be implied?)
>
> Anyways, hopefully this straw-man is able to spark some discussion, 
> and hopefully if it's not fatally flawed, I'll be able to finish 
> adopting it to the W3C template for proper and ongoing discussions.
>
> Cheers,
> Ryan
>
> References:
> [1] http://www.w3.org/TR/WebIDL
> [2] http://www.w3.org/2001/06/manual/
> [3] http://lists.w3.org/Archives/Public/public-webcrypto/2012Jun/0050.html
> [4] http://lists.w3.org/Archives/Public/public-webcrypto/2012Jun/0007.html
> [5] http://lists.w3.org/Archives/Public/public-webcrypto/2012May/0070.html
> [6] http://www.khronos.org/registry/typedarray/specs/latest/
> [7] http://www.w3.org/TR/FileAPI/
> [8] http://www.w3.org/TR/2009/WD-websockets-20091222/
> [9] http://dev.w3.org/2011/webrtc/editor/webrtc.html
Received on Thursday, 21 June 2012 08:35:36 GMT

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