Suppose I obtain the ability to access someone else's cryptocurrency.
This sounds like fraud
e.g. I overheard them saying the password to their wallet out loud or I am a custodian of their assets.
Nope, STILL fraud, possibly even Computer Misuse aka hacking... and because you use internet: it's Wire Fraud
I now borrow those assets, keep them for some period of time, and then return them, without the owner's consent.
hmmm, let's take california... Luckjy you, it is not embezzlement because you were not entrusted with the cryptocurrency, you gave yourself access.
503. Embezzlement is the fraudulent appropriation of property by a person to whom it has been intrusted.
No it is plainly... theft under California law:
484. (a) Every person who shall feloniously steal, take, carry, lead, or drive away the personal property of another, or who shall fraudulently appropriate property which has been entrusted to him or her, or who shall knowingly and designedly, by any false or fraudulent representation or pretense, defraud any other person of money, labor or real or personal property, or who causes or procures others to report falsely of his or her wealth or mercantile character and by thus imposing upon any person, obtains credit and thereby fraudulently gets or obtains possession of money, or property or obtains the labor or service of another, is guilty of theft. In determining the value of the property obtained, for the purposes of this section, the reasonable and fair market value shall be the test, and in determining the value of services received the contract price shall be the test. If there be no contract price, the reasonable and going wage for the service rendered shall govern. For the purposes of this section, any false or fraudulent representation or pretense made shall be treated as continuing, so as to cover any money, property or service received as a result thereof, and the complaint, information or indictment may charge that the crime was committed on any date during the particular period in question. The hiring of any additional employee or employees without advising each of them of every labor claim due and unpaid and every judgment that the employer has been unable to meet shall be prima facie evidence of intent to defraud.
Giving it back doesn't matter. The person taking the crypto for any amount of time without being entitled to them is committing theft. You see, California doesn't interest that you just want to borrow. They don't even require Mens Rea for the mere taking - only for fraudulent pretense there is an intent question.
In fact, it might even be automatically Grand Theft:
484d. As used in this section and Sections 484e to 484j, inclusive:[...]
(2) “Access card” means any card, plate, code, account number, or other means of account access that can be used, alone or in conjunction with another access card, to obtain money, goods, services, or any other thing of value, or that can be used to initiate a transfer of funds, other than a transfer originated solely by a paper instrument.
484e. (a) Every person who, with intent to defraud, sells, transfers, or conveys, an access card, without the cardholder’s or issuer’s consent, is guilty of grand theft.
YIKES! and now, intent to defraud comes in, but that is actually minimal: that just means taking without being allowed to by the owner in many cases.