"The recent breakdown by bitcoin and [ether] below important support near $18,431 and $1,256, respectively, confirms they are in new intermediate-term downtrends," analyst Javed Mirza said in a note Thursday.
"This is a strong technical negative and opens the door for a test of major support near $11,918 and $560, respectively, or another 30% and 57% downside from current levels."
On Wednesday, bitcoin crossed over the $17,000 level for the first time in two weeks, according to Coin Metrics.
That could take bitcoin and ether higher by 27% and 15%, respectively.
"The crypto industry is rhyming with the 1990s internet craze," Mirza said.